Extract filetype-specific setups to ftplugin

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.

vim/ftplugin/css.vim

@@ -0,0 +1 @@
+setlocal iskeyword+=-

vim/ftplugin/gitcommit.vim

@@ -0,0 +1,3 @@
+" Automatically wrap at 72 characters and spell check commit messages
+setlocal textwidth=72
+setlocal spell

vim/ftplugin/markdown.vim

@@ -0,0 +1,5 @@
+" Enable spellchecking
+setlocal spell
+
+" Automatically wrap at 80 characters
+setlocal textwidth=80

vim/ftplugin/sass.vim

@@ -0,0 +1 @@
+setlocal iskeyword+=-

vim/ftplugin/scss.vim

@@ -0,0 +1 @@
+setlocal iskeyword+=-

vimrc

@@ -44,19 +44,6 @@ augroup vimrcEx
   autocmd BufRead,BufNewFile Appraisals set filetype=ruby
   autocmd BufRead,BufNewFile *.md set filetype=markdown
   autocmd BufRead,BufNewFile .{jscs,jshint,eslint}rc set filetype=json
-
-  " Enable spellchecking for Markdown
-  autocmd FileType markdown setlocal spell
-
-  " Automatically wrap at 80 characters for Markdown
-  autocmd BufRead,BufNewFile *.md setlocal textwidth=80
-
-  " Automatically wrap at 72 characters and spell check git commit messages
-  autocmd FileType gitcommit setlocal textwidth=72
-  autocmd FileType gitcommit setlocal spell
-
-  " Allow stylesheets to autocomplete hyphenated words
-  autocmd FileType css,scss,sass setlocal iskeyword+=-
 augroup END
 
 " Softtabs, 2 spaces