Support dependency injection for routes engine

Allow a user to define which Rails engine to define routes on when using
multiple engines within a Rails application. This will ensure that path
helpers will correctly be available in the desired engine. This will
default to the default main Rails application if another engine is not
specified.

Resolves issue [#138](https://github.com/thoughtbot/high_voltage/issues/138)
Add entry to README for engine injection
Update NEWS for Engine injection