Subscriptions-based tracking

[MrSaints]

I think there's potential to expand the functionality of this script.

I think it'll be great to allow subscriptions to be added, and persisted in the brain. It will correspond to the user who added it. The user can then run a command to return a list of videos from their subscribed channels.

I can't really estimate the complexity at this point, but worst case scenario, it might only be achievable through several async API calls, and this can get a bit messy (much like hubot-twitch hacky async calls).