pr-pull: allow to specify tap

[dawidd6]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
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• Have you successfully run brew tests with your changes locally?

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This PR makes it possible to specify the tap, different than default homebrew/core.

cc @jonchang

[request-info]

Please provide a better issue/pull request title and/or description!

[jonchang]

Looks good!

[MikeMcQuaid]

Thanks again @dawidd6!