Add dhall, closes #426

ftplugin/dhall.vim

@@ -0,0 +1,39 @@
+if exists('g:polyglot_disabled') && index(g:polyglot_disabled, 'dhall') != -1
+  finish
+endif
+
+if exists('b:dhall_ftplugin')
+	finish
+endif
+let b:dhall_ftplugin = 1
+
+setlocal commentstring=--\ %s
+
+set smarttab
+
+if exists('g:dhall_use_ctags')
+    if g:dhall_use_ctags == 1
+        augroup dhall
+            autocmd BufWritePost *.dhall silent !ctags -R .
+        augroup END
+    endif
+endif
+
+function! StripTrailingWhitespace()
+    let myline=line('.')
+    let mycolumn = col('.')
+    exec 'silent %s/  *$//'
+    call cursor(myline, mycolumn)
+endfunction
+
+if exists('g:dhall_strip_whitespace')
+    if g:dhall_strip_whitespace == 1
+        augroup dhall
+            au BufWritePre *.dhall silent! call StripTrailingWhitespace()
+        augroup END
+    endif
+endif
+
+augroup dhall
+    au BufNewFile,BufRead *.dhall setl shiftwidth=2
+augroup END