depreacte enabled_for method missing
Arturo: better respond_to? handling

 1. allow a second, optional argument for `Arturo.respond_to?`
 2. also define `Arturo.respond_to_missing?` for `.method` support
 3. emit a deprecation warning when calling
    `Arturo.respond_to?` with `FEATURE_enabled_for?`
Merge branch 'deprecate-missing'