formula_creator: autodetect license from GitHub when available

[dtrodrigues]

• 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?

---

When a user provides a GitHub repository as the url when creating a new formula, brew currently retrieves the desciption and homepage to pre-populate the formula. This change will now also check to see if a license is present, and if it is, add that to the formula.

[MikeMcQuaid]

Thanks @dtrodrigues, good idea!