Add bones-stale-branches command

[tbekolay]

This command lists all stale branches in a repository. If you're in a repository with a .nengobones.yml, it will use the provided information. You can also pass a repo on the command line.

Some known issues:

• In order to avoid rate limiting, you must provide a Github username and personal access token as environment variables. This could be done in a nice way, like using the full OAuth v2 workflow.
• The call to the list of branches is not properly paginated,so only the first chunk of branches is fetched properly.

Motivation and context:
In looking at nengo/nengo#1485 I wondered if this is the kind of thing we should do regularly (remove stale branches). Since we already have the bones-pr-number script, I thought this might be pretty straightforward. Unfortunately it was not, so before putting more time into it, we should figure out if this is something we actually want.

How has this been tested?
Ran the command for Nengo Bones and Nengo Core.

How long should this take to review?

• Average (neither quick nor lengthy)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

[hunse]

It's straightforward to view stale branches in Github. Is that sufficient?

[tbekolay]

The idea with something like this is that we would use it as part of an automated process. But yeah not necessarily convinced it's a process we care about at this point.