Support custom formats for label timestamps

[lehrblogger]

More info in the associated issue https://github.com/rosulek/gmail-tickler/issues/21. I realize we could probably use the library elsewhere as well, but I wanted to minimize the changes for now.

I went back and forth a bit about whether we wanted the minified moment.js or not, but since we don't need to optimize for page load times, I figured it better for it to be readable.

This is with a format of "YYYY-MM-DD HH:mm":

[lehrblogger]

Oh, and I think we don't need Moment Timezone because it's handled by the environment in which Google runs the script, so that simplifies things a bit.

[lehrblogger]

A coworker told me about Google Apps Script Libraries today: https://developers.google.com/apps-script/guide_libraries. I think that's probably overkill for the moment.gs file, but it's still a potentially-useful thing to keep in mind.