Improve autocmd handling

Thes autocommands should be in a group that clears themselves when the
file is re-sourced so we don't re-source things and cause duplicate
events.

Author: amadeus

ftdetect/javascript.vim

@@ -1,8 +1,11 @@
-au BufNewFile,BufRead *.{js,mjs,jsm,es,es6},Jakefile setf javascript
-
 fun! s:SelectJavascript()
   if getline(1) =~# '^#!.*/bin/\%(env\s\+\)\?node\>'
     set ft=javascript
   endif
 endfun
-au BufNewFile,BufRead * call s:SelectJavascript()
+
+augroup javascript_syntax_detection
+  autocmd!
+  autocmd BufNewFile,BufRead *.{js,mjs,jsm,es,es6},Jakefile setfiletype javascript
+  autocmd BufNewFile,BufRead * call s:SelectJavascript()
+augroup END