Cleanup .vimrc

1. Change Go's <leader>gd to match vim's gd. Now (for Go files),
   <leader>gd => go-def, and <leader>gh => go-doc
2. Breakup couple of lines (in augroup code_langs) for formatting
3. Add shortcut F12 (disabled by default) to toggle between light/dark
   background. Useful for colorschemes like solarize and base16-*