Adds an `in` option

Allows for cleaner environment-centric corralling (shout out to Paddock)

Example:

```ruby
corral(Rails.env) do
  disable :caching, in: :production
  disable :verbose_logs, in: [:staging, :production]
end
```
This doesn't make sense as a public API method
Coerce environment to symbol
Refactors for clearer division of internal/public methods
Moves Corral::Feature to its own file
Cleanup
README updates
More accurate docs
Merge pull request #2 from bryanwoods/environment_support

Adds an `in` option