Add NecoGhcDiagnostics command

ISSUES.md

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+# neco-ghc issues
+
+When submitting an issue, please include the output by `:NecoGhcDiagnosis`.
+
+```vim
+NecoGhcDiagnosis
+```
+
+It shows your environment information possibly related to neco-ghc.
+
+- Current filetype
+    - neco-ghc only works in the buffer with filetype haskell.
+- ghc-mod executable (required)
+    - neco-ghc requires [ghc-mod](https://github.com/kazu-yamamoto/ghc-mod) and it must be placed in your `$PATH`.
+- 'omnifunc'
+    - To use Vim builtin completion, `'omnifunc'` must be set to `necoghc#omnifunc` by `setlocal omnifunc=necoghc#omnifunc`.
+- Completion plugin installation (optional)
+    - [neocomplete.vim](https://github.com/Shougo/neocomplete.vim)
+    - [neocomplcache.vim](https://github.com/Shougo/neocomplcache.vim)
+    - [YouCompleteMe](https://github.com/Valloric/YouCompleteMe)
+- Other  plugin installation (optional)
+    - [vimproc.vim](https://github.com/Shougo/vimproc.vim)

autoload/necoghc/diagnostics.vim

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+function! necoghc#diagnostics#report()
+  echomsg 'Current filetype:' &l:filetype
+
+  let l:executable = executable('ghc-mod')
+  echomsg 'ghc-mod is executable:' l:executable
+  if !l:executable
+    echomsg '  Your $PATH:' $PATH
+  endif
+
+  echomsg 'omnifunc:' &l:omnifunc
+  echomsg 'neocomplete.vim:' exists(':NeoCompleteEnable')
+  echomsg 'neocomplcache.vim:' exists(':NeoComplCacheEnable')
+  echomsg 'YouCompleteMe:' exists(':YcmDebugInfo')
+
+  try
+    echomsg 'vimproc.vim:' vimproc#version()
+  catch /^Vim\%((\a\+)\)\=:E117/
+    echomsg 'vimproc.vim: not installed'
+  endtry
+endfunction

plugin/necoghc.vim

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+command! -nargs=0 NecoGhcDiagnostics call necoghc#diagnostics#report()