Remove `request.features`

Decorating the `request` object causes issues if a Reflip-enabled Express app mounts a Reflip-enabled sub-app with different settings. If app A configures Reflip with `"aardvarks"` enabled, app B configures an independent Reflip instance with `"aardvarks"` disabled, and A mounts B as a sub-app, `request.features.aardvarks` will be `false` if A mounts B after `reflip.flip()`, and `true` if A mounts `.flip()` after B.

The internal cache of feature lookups is now exposed by calling `request.check()` without any arguments. Mutating this object will affect the results returned by `request.check('feature-name')`, consistent with the current behavior.

This is a backward-incompatible change, though. What do you think, @ceejbot?