Support YouTube handles

[spedrickson]

Now that YouTube supports @handles for accounts, it would be valuable for the ytchat-ingest tool to support them as well.

Current impact of using a handle with the tool:
Currently the only downside of launching the tool with a @handle instead of a channelID is messages will be put in a collection named after the handle rather than the channelID.

Workaround:
Just launch the tool with a channelID (UC...) instead of a @handle

Long-term fix:
For consistency, the tool should resolve @handles into channelIDs on launch, and use that as the collection name. This should also be more stable against end-user changes like new handles, since channelIDs are unchangeable.