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

autoload/lsp/ui/vim/output.vim

@@ -0,0 +1,38 @@
+function! lsp#ui#vim#output#preview(data) abort
+    " Close any previously opened preview window
+    pclose
+
+    execute &previewheight.'new'
+
+    let l:ft = s:append(a:data)
+    " Delete first empty line
+    0delete
+
+    setlocal readonly nomodifiable
+
+    let &l:filetype = l:ft . '.lsp-hover'
+endfunction
+
+function! s:append(data) abort
+    if type(a:data) == type([])
+        for l:entry in a:data
+            call s:append(entry)
+        endfor
+
+        return 'markdown'
+    elseif type(a:data) == type('')
+        put =a:data
+
+        return 'markdown'
+    elseif type(a:data) == type({}) && has_key(a:data, 'language')
+        put ='```'.a:data.language
+        put =a:data.value
+        put ='```'
+
+        return 'markdown'
+    elseif type(a:data) == type({}) && has_key(a:data, 'kind')
+        put =a:data.value
+
+        return a:data.kind == 'plaintext' ? 'text' : a:data.kind
+    endif
+endfunction