Remove some redundancy in the "skel" logic.
szakdark: adjust the diff color to remove bolding and allow highlighting

The bolding of added text in the console was a bit much, and the colors
overall needed some tweaking.
Break undo when triggering a snippet.
Document how to run vimrc unit tests.
- Add UltiSnips maps to jump forward or list snippets:

  - <M-u><Tab> jumps forward without attempting a snippet expansion.

  - <M-u><M-l> lists snippets at cursor.

- Break undo when triggering a snippet, so that if you under after
  expanding a snippet, only the snippet expansion is undone.
Normalize case in keys (e.g., <C-X> => <C-x>).

This is in anticipation of someday distinguishing <C-x> from
<C-X> (as happens already for <M-x> and <M-X>).
Merge branch 'szakdark-diff-colors' of https://github.com/jszakmeister/vimfiles