Add pr-upload command

[dawidd6]

As a replacement for brew test-bot --ci-upload and per suggestion on Slack.

• 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.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

Looks good! Could you add a simple _spec.rb file (that just ensures the arguments can be parsed)?

[MikeMcQuaid]

Thanks @dawidd6!

[sjackman]

Yes, thank you, Dawid!

Opened PR #7406 to add argument --root-url needed by third party taps.