Provide granular feed customization

[fluffy-critter]

The Atom feeds are kind of one-size-fits-none right now. It'd be great to add some query parameters like:

• subcats -- whether to include subcategories (defaults to True)
• ignore -- ignore a category

e.g. someone should be able to subscribe to http://beesbuzz.biz/feed?ignore=blog/chatter&ignore=art/drawings/sketchbook, or subscribe specifically to http://beesbuzz.biz/art/feed?subcats=0 or something.

It'd also be neat to have a little subscription builder thing that people can consult to produce a feed URL for them. Not sure how to make that discoverable though. Maybe a subscribe.html template which is linked to from the category index?

[fluffy-critter]

Prerequisite: PlaidWeb/Publ#215

Also would be helpful to have PlaidWeb/Publ#187