FEATURE: site setting for Markdown typographer

It ships anyway with markdown.it so we might as well expose it

app/assets/javascripts/pretty-text/engines/discourse-markdown-it.js.es6

@@ -133,7 +133,7 @@ export function setup(opts, siteSettings, state) {
     breaks: opts.discourse.features.newline,
     xhtmlOut: false,
     linkify: true,
-    typographer: false
+    typographer: siteSettings.enable_markdown_typographer
   });
 
   setupHoister(opts.engine);