allows users to pass broadcaster IDs into verify-subscription

[lleadbet]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

#103 identified a gap in the ability to pass the broadcaster's user ID into the subscription payload, which this now enables.

Description of Changes:

• Adds --broadcaster and -b to the verify-subscription subcommand which is passed to the subscription body.

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)