Improve matching parent group style for cterm

A more detailed review of this change can be found in the PR thread:
https://github.com/arcticicestudio/nord-vim/pull/78#pullrequestreview-85806310

The changes from the previous commit are good, but there haven still
been some edge cases which had to be coverd. It applied for the "gui"
mode or when the "termguicolors" option is set, but the "cterm" mode is
still affected by the problem (1).

When changing the cterm definitions it had to be keept in mind that there
is no "nord0_term" variable. If set to "NONE" (to make it kind of
"transparent" and use the main background color) there is another edge
case when the "cursorline" option is enabled so only the foreground color
of the matching elements would change to "nord8" (2, 3).

To optimally improve the highlighting for both "gui" and "cterm" mode
"nord3" will be used as background color for the matching element which
doesn't conflict with the "cursorline" color and also stands out in order
to see the matching element (4, 5, 6)

References:

  (1) https://user-images.githubusercontent.com/7836623/34409455-3370fada-ebca-11e7-9ed8-d83c3a9c5055.png
  (2) https://user-images.githubusercontent.com/7836623/34409493-6221fdd4-ebca-11e7-97c4-703ca9e5198a.png
  (3) https://user-images.githubusercontent.com/7836623/34409508-74f913a2-ebca-11e7-8dd2-896ac809e23b.png
  (4) https://user-images.githubusercontent.com/7836623/34409567-d8c64bfc-ebca-11e7-919a-1a40cf146084.png
  (5) https://user-images.githubusercontent.com/7836623/34409570-dec9c1aa-ebca-11e7-8e68-32b79ec0d7c9.png
  (6) https://user-images.githubusercontent.com/7836623/34409581-eddcefc8-ebca-11e7-9ab7-29b80704d836.gif

GH-75

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