Remove RSS generator plugin

This plugin is not supported by Github Pages, where this blog is hosted
and the link to an RSS feed was removed in
bcfb48df8d884c2a3254026d3391ef9c909bfad2.
Add RSS feed at /feed.xml

Add an RSS feed using a Liquid template to render XML. Based from the
`feed.xml` file included in Jekyll 2.x's default theme (`jekyll new .`).

Adapted to use `site.name` rather than `site.title`.
Partically revert "Remove RSS subscription link"

This essentially reverts commit
bcfb48df8d884c2a3254026d3391ef9c909bfad2, but changes the link to the
RSS feed from `rss.xml` to `feed.xml`.