Support for validation of PR body OR title OR commits

[ineffyble]

Is your feature request related to a problem? Please describe.
As a team leader, I want to encourage my team to link their work to Rally artifacts, without being prescriptive in how they do this.

Describe the solution you'd like
There should be an option to validate that a Rally artifact is present in one of the supported fields.

[github-actions]

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[ineffyble]

This should stay open.

[JoshuaTheMiller]

How about making this work similar to Flag Enums in C#. Users could set a single value, locationsToCheck and pass an array of the locations they would want this bot to check. This could also help satisfy #121 if it was deemed something this Probot would like to support.

An additional option could be added to change how many locations are required (to continue the existing and-ish behavior): minimumLocations (the name could be made better...).

Example

# .github/rally.yml
locationsToCheck:
- prTitle
- prBody
# - commit
# - branch

minimumLocations: 1