This extracts the Readline support into a separate class and keeps only the generic behavior in the main interactor class, thus enabling the return of the previous simple `#gets` based interactor. Guard auto detects if Readline support is suitable on your environment, but you can also use the new `interactor` DSL method to force a specific implementation. (Fixes #197 and #200)
Conflicts: README.md guard.gemspec
I know this smells and I should write better specs so that this isn't necessary, but for my given time limit to work on Guard, this is the best solution for now.