Add YouTube channel IDs

Adds YouTube channel IDs, as discussed in #80.  

Also deals with a few tough cases where the YouTube URL that we scraped was actually a redirect or alias (and thus unusable in any of Google's APIs).

One breaking change: Accounts that do not have a YouTube username no longer have their `youtube` field populated (previously, it was populated with `channel/[id]`).  This is more consistent, but could break some existing scripts.

Be warned: I'm usually not a Python guy, so this code might be rubbish.