A bunch of extensions and config meta pages work

Here's a handful of commits from tonight having to do with the new extension class and the config meta pages. Highlights:

* Ported `:asset_hash` extension to the new system.
* Config meta page now lists active and inactive extensions and options details for new-style extensions.
* Users can now modify extension configuration after `activate` by accessing `extensions[:asset_hash].options`.
* If a user passes a setting that an extension hasn't declared with `option`, they'll get an error.
* Meta pages render even if `:asset_hash` is activated. :neutral_face:
* I couldn't think of a way to make the meta pages discoverable, so I'm just printing out their URL to the console on startup.
* The rack app is cached in a more effective way, and the `:asset_hash` extension doesn't recreate the rack app a million times anymore.

The meta pages are still butt-ugly - I plan on giving them a nice style maybe based on the guides site, but I wouldn't mind some help.