Add CLI tool for consistent label management across repos

[mbrukman]

Since CRBot can add labels like cla: yes, cla: no and cla: external to a PR (if those labels exist), it would be nice to ensure both the existence of labels, as well as ensure that their colors are consistent across repos, and optionally, that they have descriptions (also consistent across repos).

If we generalize it further, it would be nice to have a tool for managing common labels across repos for ensuring consistency: that the labels exist, they have the same colors, and descriptions.

The configuration can be done in a simple language such as YAML or JSON as we don't need much in the way of programmability, and it would enable us to avoid having to manually add these labels to new repos when spinning up a new project or a subrepo of an existing organization/project.

We may be able to factor out some label-handling code from the current ghutil package into a new label management package for reuse in this new tool.