fallbacks

Hold configured generators fallbacks. If a plugin developer wants a generator group to fallback to another group in case of missing generators, they can add a fallback.

For example, shoulda is considered a test_framework and is an extension of test_unit. However, most part of shoulda generators are similar to test_unit ones.

Shoulda then can tell generators to search for test_unit generators when some of them are not available by adding a fallback:

Rails::Generators.fallbacks[:shoulda] = :test_unit