Add vertical to 'diffopt' setting
Add mappings to generate markdown headings in insert mode
Remove q mapping from scratch buffer. <C-q> will suffice.
Modify MegaTab to work with <S-Tab> by taking keymappings as args

- Also fix column number bug.  Column numbers were off by one and causing
  MegaTab to return the wrong keymapping.
Add NextIndent function and mappings
Update Fugitive mappings
Fix bug in FixGitGutterSignColumn when changing colorschemes.

- Solarized and hybrid would have their signs corrupted because other
  colorschemes set the highlight groups directly.  This caused the
  GitGutterAdd/Change/Delete groups to be reset and no longer link to their
  default groups.
Add SnippetSyntax function to highlight code snippets in markdown

- Add autocmd to call function when loading pandoc or markdown filetype.
Move colorscheme autocmd functions into SetupColorScheme function
Add ExtraWhitespace hi group and SetCustomHL function
Move Sneak highlighting into SetCustomHL()
Reorganize mappings section