Improvements for `chef prepare`:
- `Chef` now has a custom `Environment`. This required a few changes in how
  environments are created and initialized.
- Added concept of "command extension", which is pretty much a sub-command.
- The different types of preparations are in their own command extension.
- Added a `prepare feed` command extension to create RSS/Atom feeds.

Miscellaneous:
- Renamed the `xmldate` filter to `atomdate`.
- Added a `pcfail` function to fail the bake or rendering on purpose.
Updated CHANGELOG.