Use sbt.scala multi-filetype for sbt files

So we don't stomp on vim-sbt. This also allows other goodies to work
without necessarily having vim-sbt installed:

  - Loading snippets for sbt from a dotted filetype-aware snippets
    plugin, like UltiSnips.

  - Disabling Syntastic for sbt filetype since the syntax check results
    are not useful.

  - Everything else you can do with FileType autocommands.

Closes #54

ftdetect/scala.vim

@@ -4,5 +4,8 @@ fun! s:DetectScala()
     endif
 endfun
 
-au BufRead,BufNewFile *.scala,*.sbt set filetype=scala
+au BufRead,BufNewFile *.scala set filetype=scala
 au BufRead,BufNewFile * call s:DetectScala()
+
+" Install vim-sbt for additional syntax highlighting.
+au BufRead,BufNewFile *.sbt setfiletype sbt.scala