Support for user-configurable feed templates.

[fancypantalons]

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.

---

My use case for this one is integration with micro.blog while wanting to continue to support my regular RSS feed for normal subscribers. I've created a separate RSS feed containing only my notes (putting them all in the same category), and I want to exclude the entry title from each element in the feed so micro.blog treats them as notes instead of articles.

However, I still want a normal full feed with everything else in it.

This change allows me to alter the feed structure for the notes only.

I noticed there's a number of other requests to be able to specify alternative titles by category and so forth, and those requests could all be covered by this same change.

In addition, I noticed another PR that requested being able to generate multiple of the same feed with different layouts. I could see altering this changeset to enable that kind of use case through some alterations to the config structure if that's useful.