Implement uniform diff background theme config

Added a theme configuration to let the users decide whether they want
to use colorful or uniform diff backgrounds.

The colorful diff is the default using the current colors for added,
changed and deleted elements and the default background color (nord0).
The "reverse" attribute gets applied which results in a colored
background and a dark foreground.

Setting the "g:nord_uniform_diff_background" variable to "1" enables
the uniform diff background using "nord1".

GH-60

4 files changed, 29 insertions, 10 deletions

diff --git a/README.md b/README.md
index 4917782..707d713 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,7 @@ Nord Vim is a 16 colorspace theme build to run in GUI- and terminal mode with su
   - [Configuration](#configuration)
     - [Italic comments](#italic-comments)
     - [Comment contrast](#comment-contrast)
+    - [Uniform diff background](#uniform-diff-background)
   - [Plugin Support](#plugin-support)
     - [UI Plugins](#ui-plugins)
     - [Language Plugins](#language-plugins)
@@ -132,8 +133,20 @@ To adhere to the Nord style guide this option uses `nord3` by default.
 
 More information about true color and the support in various terminals can be found in [this gist][gist-true-color].
 
+### Uniform diff background
+By default, Nord Vim provides colorful backgrounds when used in *diff* mode `vimdiff`/`vim -d`.
+
+This can be changed to `nord1` as uniform *diff* background color by setting the `g:nord_uniform_diff_background` variable to `1`.
+
+```vim
+let g:nord_uniform_diff_background = 1
+```
+
+<align="center"><strong>Colorful backgrounds (default)</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-config-uniform-diff-default.png"/><br><strong>Uniform diff background</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-config-uniform-diff-default.png"/>
+</p>
+
 ## Plugin Support
-Nord Vim provides support for many third-party language- and the UI plugins.  
+Nord Vim provides support for many third-party language- and UI plugins.
 
 ### UI Plugins
 <p align="center"><a href="https://github.com/itchyny/lightline.vim">lightline</a><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-plugin-support-ui-lightline.png"/><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-plugin-support-ui-lightline-vsplit.png"/></p>
diff --git a/assets/scrot-config-uniform-diff-default.png b/assets/scrot-config-uniform-diff-default.png
new file mode 100755
index 0000000..db96f9a
--- /dev/null
+++ b/assets/scrot-config-uniform-diff-default.png
diff --git a/assets/scrot-config-uniform-diff.png b/assets/scrot-config-uniform-diff.png
new file mode 100755
index 0000000..16b9aae
--- /dev/null
+++ b/assets/scrot-config-uniform-diff.png
diff --git a/colors/nord.vim b/colors/nord.vim
index 9df083e..9779dc4 100755
--- a/colors/nord.vim
+++ b/colors/nord.vim
@@ -80,6 +80,10 @@ if !exists("g:nord_comment_brightness")
   let g:nord_comment_brightness = 0
 endif
 
+if !exists("g:nord_uniform_diff_background")
+  let g:nord_uniform_diff_background = 0
+endif
+
 function! s:hi(group, guifg, guibg, ctermfg, ctermbg, attr, guisp)
   let l:attr = a:attr
   if g:nord_italic_comments == 0 && l:attr ==? 'italic'
@@ -261,15 +265,17 @@ hi! link dtDelim Delimiter
 hi! link dtLocaleValue Keyword
 hi! link dtTypeValue Keyword
 
-call s:hi("diffAdded", s:nord14_gui, "", s:nord14_term, "", "", "")
-call s:hi("diffChanged", s:nord13_gui, "", s:nord13_term, "", "", "")
-call s:hi("diffNewFile", s:nord8_gui, "", s:nord8_term, "", "", "")
-call s:hi("diffOldFile", s:nord7_gui, "", s:nord7_term, "", "", "")
-call s:hi("diffRemoved", s:nord11_gui, "", s:nord11_term, "", "", "")
-call s:hi("DiffAdd", s:nord14_gui, "", s:nord14_term, "", "", "")
-call s:hi("DiffChange", s:nord13_gui, "", s:nord13_term, "", "", "")
-call s:hi("DiffDelete", s:nord11_gui, "", s:nord11_term, "", "", "")
-call s:hi("DiffText", s:nord4_gui, "", "NONE", "", "", "")
+if g:nord_uniform_diff_background == 0
+  call s:hi("DiffAdd", s:nord14_gui, s:nord0_gui, s:nord14_term, "NONE", "inverse", "")
+  call s:hi("DiffChange", s:nord13_gui, s:nord0_gui, s:nord13_term, "NONE", "inverse", "")
+  call s:hi("DiffDelete", s:nord11_gui, s:nord0_gui, s:nord11_term, "NONE", "inverse", "")
+  call s:hi("DiffText", s:nord13_gui, s:nord0_gui, s:nord13_term, "NONE", "inverse", "")
+else
+  call s:hi("DiffAdd", s:nord14_gui, s:nord1_gui, s:nord14_term, s:nord1_term, "", "")
+  call s:hi("DiffChange", s:nord13_gui, s:nord1_gui, s:nord13_term, s:nord1_term, "", "")
+  call s:hi("DiffDelete", s:nord11_gui, s:nord1_gui, s:nord11_term, s:nord1_term, "", "")
+  call s:hi("DiffText", s:nord13_gui, s:nord1_gui, s:nord13_term, s:nord1_term, "", "")
+endif
 
 call s:hi("gitconfigVariable", s:nord7_gui, "", s:nord7_term, "", "", "")