Allow label renaming

[larsks]

If I have a label bug and I would like it to be type: bug, I wish I could do something like this:

- name: "type: bug"
  alias: "bug"

And then label-syncer would:

• Create a new label type: bug if no label named either type: bug or bug exists, or
• Rename an existing label bug to be type: bug

This would make it much easier to make a set of existing repositories consistent.

[joeyparrish]

I found it necessary to extend this original idea a bit. To support a single config to get multiple, divergent repos using the same labels, I need support for multiple aliases for some:

- name: "Type: bug"
  aliases:
    - bug
    - defect
  description: Something isn't working
  color: d73a4a

I think we should support both syntaxes (single alias and multiple). But having support for either a string or an array of them in the parser seems to require two different field names (alias as a string, and aliases for an array of strings).