feat(colors): add `CmpItemMenuThumb` highlight

ref(colors): `Compe` -> `Cmp`

`nvim-compe` was deprecated.

feat(colors): gitsigns.nvim support

ref(colors): make non-error messages less menacing

Merge pull request #10 from alex-popov-tech/patch-1

Update README.md

ref(colors): extract help groups

There were help syntax highlights scattered around. THis moves them to
one, dedicated spot.

Additionally, there were some incorrect settings for markdown help
blocks, and they were corrected.

Fix vim-sandwhich diff text

Document new functionality

Improve vimscript support

Add docs