Add GitGutter/GitSigns highlighting (#234)

This adds specific highlight groups for GitGutter-related things.
It piggybacks off of the `Diff*` highlight groups.

The `gitsigns.nvim` maintainer [suggested](https://github.com/lewis6991/gitsigns.nvim/issues/94) that this be
implemented by checking if neovim is >= version 0.5, and if the package is
available (as opposed to a variable like `g:loaded_gitsigns`). This is because
`gitsigns` is essentially lazy-loaded so checking the variable when Dracula loads might be too early.

See these other related links in from [gitsigns](https://github.com/lewis6991/gitsigns.nvim):
https://github.com/lewis6991/gitsigns.nvim/issues/54
317750d66a

after/plugin/dracula.vim

@@ -41,6 +41,26 @@ if exists('g:loaded_ctrlp')
   hi! link CtrlPBufferHid Normal
 endif
 " }}}
+" GitGutter / gitsigns: {{{
+if exists('g:loaded_gitgutter')
+  hi! link GitGutterAdd    DiffAdd
+  hi! link GitGutterChange DiffChange
+  hi! link GitGutterDelete DiffDelete
+endif
+if has('nvim-0.5') && luaeval("pcall(require, 'gitsigns')")
+  " https://github.com/lewis6991/gitsigns.nvim requires nvim > 0.5
+  " has('nvim-0.5') checks >= 0.5, so this should be future-proof.
+  hi! link GitSignsAdd      DiffAdd
+  hi! link GitSignsAddLn    DiffAdd
+  hi! link GitSignsAddNr    DiffAdd
+  hi! link GitSignsChange   DiffChange
+  hi! link GitSignsChangeLn DiffChange
+  hi! link GitSignsChangeNr DiffChange
+  hi! link GitSignsDelete   DiffDelete
+  hi! link GitSignsDeleteLn DiffDelete
+  hi! link GitSignsDeleteNr DiffDelete
+endif
+" }}}
 " Tree-sitter: {{{
 if exists('g:loaded_nvim_treesitter')
   " # Misc