Implement tool to synchronise labels across all repos in an org

[iphydf]

$ hub-sync-labels TokTok c-toxcore

The above command should take all the issue labels from the TokTok/c-toxcore repository and synchronise them to all other repositories in the TokTok organisation. It should:

• delete labels that are no longer there,
• add labels that were added,
• sync label colours,
• have a dry-run mode (show me what it would do, but don't actually do it),
• and optionally: have an option to only add new labels but don't delete old ones.