feat(templates): add support for custom template URLs

[dkundel]

This fixes #113. I added some basic tests for now. I'll add some additional tests before merging but I'd love your eyes at this @philnash

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[philnash]

Looks good. Only thing I can think of is handling the case where a user supplies a branch or repo that doesn't exist or isn't accessible (private repo issues?).

[dkundel]

I think for now we'll just do the normal error handling here. I'll file an issue for improved error handling though

[philnash]

Sounds good. Let's get this in.