A settings option to enable calendar subscription via .ics
First pass at rendering the .ics feed. It includes some part of that, plus abstraction of filter validation and generation of each post's supplementary information.

It still needs:

* Proper validation that the request is acceptable.
* Feed should actually render in iCal.
* UI for the user to access their personalized feed.

For security purposes, the feed should include a key that's MD5 hash of the user's user_login and a secret key generated on activation and stored in options.