Add ability to start a generator with an empty repo or Git repo

[johnsonr]

Backward compatible. Solves 2 important cases:

• It's now possible to start with an empty repo, regardless of the content of a generator's Rug archive. This is useful when emitting content programmatically, rather than transforming static content.
• It's now possible to start with a git reference. This means we can point a generator at any accessible repo.

[Lawouach]

I love the idea and what we could do with it.

Looking at the code, I have a couple of questions:

• what is the defaultProject taken by the startingPoint method? Why is it needed?
• is the naming startingPoint the right one for that method? Maybe, generateFromProject?

[johnsonr]

• The defaultProject is the project that otherwise would be the starting point. From the rug archive
• I prefer startingPoint as it can be an entirely new starting point, not necessarily based on the default.

[kipz]

LGTM