Add support for Mermaid. Resolves #1118. (#1834)

* Add Mermaid node package
* Add mermaid wiki setting and load mermaid conditionally
* Perform js error checks with mermaid active
* Precompile static assets

bin/gollum

@@ -129,6 +129,9 @@ MSG
   opts.on('--critic-markup', 'Enable support for annotations using CriticMarkup.') do
     wiki_options[:critic_markup] = true
   end
+  opts.on('--mermaid', 'Enable support for Mermaid diagrams.') do
+    wiki_options[:mermaid] = true
+  end
   opts.on('--irb', 'Launch Gollum in \'console mode\', with a predefined API.') do
     options[:irb] = true
   end