Support custom formats for label timestamps

[lehrblogger]

I'd like to add an optional setting for specifying the yyyy-MM-dd' at 'HH:mm format for the tickler-managed labels to some other sufficiently-specific value. It would be nice to be able to see the full timestamps in my narrow Gmail sidebar, perhaps by using a two-digit year:

Javascript's Date parsing is, unfortunately, terrible, and Google Apps Script's seems not much better, so this will probably be most-simply accomplished with an external dependency such as Moment and possibly Moment Timezone. While in most situations I'd cringe at the relatively-large library sizes, in this case I guess it won't really matter.

[lehrblogger]

(Sorry for all the commits referenced above, I got into a minor scuffle with git 🤕 )

[lehrblogger]

I updated the changelog and revision date over in https://github.com/rosulek/gmail-tickler/issues/20, but don't want to cause a merge conflict with the former by doing the same here. One of us can just update on master after they've been merged.