Allow resolve a template from a sub-directory

[cchantep]

Either locally or from a sub-directory in a Git repo, to make it possible having multiple (related) templates in the same repo.

[ches]

See #396

Locally, it's possible to do e.g. sbt new file://path/to/a/template

[TonioGela]

Isn't this actually implemented here @eed3si9n?

[eed3si9n]

I guess so. I'm closing this for now, but let us know if --directory doesn't address it.