Update project initializer with greater control of how it works

I'd like to eventually work towards making `RSpec::Core::ProjectInitializer` a possible public API. I'm starting to leverage this with `rspec-rails` (see rspec/rails#990). In order to handle some of the special case behavior that Rails generators require, I needed a bit more control over how the initializer worked.

- Provide ability to toggle output
- Provide ability to use an optional target directory, instead of the current working directory