First pass at ctags integration

This adds a bunch of ctag kinds that I find useful for myself.

Configuration options:
g:elm_ctags_config - Location of the .cnf file that ctags uses. By
default, it is defaulting to the one that I've added to this repo. I
added this option because people may have their own ~/.ctags file with
elm configuration (and other languages) already, which causes tagbar to
show duplicate entires.

g:elm_ctags_target_dirs - The directories that should be indexed by
ctags. By default, I made it ./elm-stuff and ./src so that people can
jump around their own source as well as the core library stuff.

g:elm_ctags_exe - Name of the ctags executable, just so people can point
it to /usr/local/bin or something.

Outstanding issues:
* No scope features
* No idea how to support signatures in ctags. I really just wanted
signatures to show up in tagbar. I was able to get it working by
changing the .cnf to create tag names like `MyFunc (A -> B)`, but then
those are then interpreted as actual tags, so you can't jump around using
<c-]> in Vim. I'm considering having two .cnf files: one for generating
the `tags` file and one for use with tagbar to get around it. The ctags
documentation mentions the --fields+S options, but it also says that it
only works for C family languages and there doesn't seem to be a way of
adding it via the `--regex-` flags.

Hack to make sure that imports don't define tags

Currently, `import Html` will make vim jump to the import statement
instead of the Html module. This hack will make sure that the tag
generated in the import statement doesn't get jumped to.

ftplugin/elm/ctags/elm.cnf

@@ -0,0 +1,11 @@
+--langdef=Elm
+--langmap=Elm:.elm
+--regex-Elm=/^ *([a-z][a-zA-Z0-9_-]+)[ \t ]*:[^:].*->(.*)/\1/f,function,functions type declarations,S:String s/
+--regex-Elm=/^ *([a-z][a-zA-Z0-9_-]+)[ \t ]*:([^:][^-]+)$/\1/c,constant,constants/
+--regex-Elm=/^port *([a-z][a-zA-Z0-9_-]+)[ \t ]*:[^:]/\1/p,port,ports/
+--regex-Elm=/^type *([A-Z][a-zA-Z0-9_-]*)/\1/t,type/
+--regex-Elm=/^type alias *([A-Z][a-zA-Z0-9_-]*)/\1/t,type,Declared types/
+--regex-Elm=/[ \r\n\t][\|=][ \r\n\t]*([A-Z][a-zA-Z0-9_-]*)(.*)/\1/C,constructor,constructors created by types/
+--regex-Elm=/^module *([A-Z][.a-zA-Z0-9_-]*)/\1/m,module,module definitions/
+--regex-Elm=/^import *([A-Z][.a-zA-Z0-9_-]*)(.*as *([A-Z][.a-zA-Z0-9_-]*))?.*(\(.*\))*/\1 (\2)/i,import,imported modules/
+

ftplugin/elm/tagbar.vim

@@ -4,11 +4,34 @@ elseif globpath(&rtp, 'plugin/tagbar.vim') == ""
     finish
 endif
 
+if !exists("g:elm_ctags_config")
+	let g:elm_ctags_config = expand('<sfile>:p:h:h') . '/elm/ctags/elm.cnf'
+endif
+
+if !exists("g:elm_ctags_target_dirs")
+	let g:elm_ctags_target_dirs = './elm-stuff ./src'
+endif
+
+if !exists("g:elm_ctags_exe")
+	let g:elm_ctags_exe = 'ctags-exuberant'
+endif
+
+function! GenerateElmTags()
+    exec system(g:elm_ctags_exe . ' -R --fields=+l --options=' . g:elm_ctags_config . ' ' . g:elm_ctags_target_dirs)
+endfunction
+command! ElmGenerateTags call GenerateElmTags()
+
+let g:Plugin_install_dir = expand('<sfile>:p:h:h')
 function! s:SetTagbar()
     if !exists("g:tagbar_type_elm")
         let g:tagbar_type_elm = {
                     \ 'ctagstype' : 'elm',
+                    \ 'deffile' : g:elm_ctags_config,
                     \ 'kinds'     : [
+                    \ 'm:module',
+                    \ 'i:imports',
+                    \ 't:types',
+                    \ 'C:constructors',
                     \ 'c:constants',
                     \ 'f:functions',
                     \ 'p:ports'