Render markdown into html in /feed.xml

`<content type="html">` in [/feed.xml](http://r7kamura.github.io/feed.xml) contains raw markdown instead of HTML.
This commit makes markdown rendered into html in `/feed.xml` (and collect up markdown setting in `app.rb`).

I tried to write a spec for it, but I cannot find a good way to tell apart markdown and HTML.