Set up spelling, format options for Markdown files
Move gitgutter settings to git.vim
Add keymap to toggle indent guides
Clean up keymap.vim

- Remove comment bloat
- Use <leader> instead of explicit ','s
- Remove '//' keymap to toggle history (already have 'coh' from
  unimpaired.vim)
Move GitGrep keymap to git.vim
Use 'syntax' and 'filetype' only when available

Inspired by http://vimcasts.org/e/20
Add DiffOrig utility command

Also, rename vimdiff.vim -> diff.vi
Remember last position per file