Simplify configuration a bit, and make extensions raise an error if the user tries to set an invalid extension option.
Update the extension template for the new Extension class.
Rewrite asset hash extension with new extension class
Save information about extensions in a discoverable way.

This allows us to display extension config information at /__middleman/config/, and also allows users to modify extension
settings after they're activated in config.rb with a statement like "extensions[:asset_hash].options.exts += ['.js']".
Reorganize Request a bit
Add in the metadata rack app early so it is there whenever middleware grabs the app
Add extensions to meta config page
Advertise meta pages on startup
Merge pull request #861 from bhollis/extensions

A bunch of extensions and config meta pages work