Create preview window instead of using quickfix list (#78)

* Create preview window instead of using quickfix list

This removes from us burden of providing formatting for content as this
is now simple as setting proper filetype.

* Create custom filetype for preview window

This will allow users to provide their own mappings and features in
hover window easily.

* Add proper statusline for LSP Hover

* Guard and undo_ftplugin

* Support all LSP hover syntaxes

ftplugin/lsp-hover.vim

@@ -0,0 +1,14 @@
+" No usual did_ftplugin header here as we NEED to run this always
+
+setlocal previewwindow buftype=nofile bufhidden=wipe noswapfile nobuflisted
+setlocal nocursorline nofoldenable
+
+if has('syntax')
+    setlocal nospell
+endif
+
+let &l:statusline = ' LSP Hover'
+
+let b:undo_ftplugin = 'setlocal pvw< bt< bh< swf< bl< cul< fen<' .
+            \ (has('syntax') ? ' spell<' : '') .
+            \ ' | unlet! g:markdown_fenced_languages'