This commit adds a new configuration setting that allows the user to
specify their own feed template, either for all feeds, or for categories
or collections, as follows:

feed:
  ...
  templates:
    -
      path: feed-foo.xml
      collection: bar
      category: baz
    -
      path: feed-xyz.xml
      collection: bar
    -
      path: feed-qwe.xml
      category: baz
    -
      path: feed.xml

For a given feed, the first matching template is the one that's used.

This change enables multiple main feeds with different layouts.