Support `'` format flag for `@printf`

[carstenbauer]

See https://stackoverflow.com/questions/52213829/in-julia-insert-commas-into-integers-for-printing-like-python-3-6.

Would it be difficult to support the ' format flag that adds thousand separators? Currently, using it explicitly throws an error that it is not yet supported.

[JimDavidson]

I concur. Adding comma insertion support would be a very useful feature to me.

[nickeubank]

agreed!