Vim's documentation advises that buffer-specific configuration based on
a filetype should be performed via a file in `$HOME/.vim/ftplugin`:
http://vimdoc.sourceforge.net/htmldoc/usr_41.html#41.12
This change moves CSS/Sass-, Git commit message-, and Markdown-related
editor settings into `ftplugin/gitcommit.vim` and
`ftplugin/markdown.vim`, and rewrites those settings to avoid using
`autocmd`, letting Vim execute `setlocal` when a file of the filetype is
determined or set.
