Add support for trailing commas in tuples

[IwanKaramazow]

Implements #889

This does not print trailing commas when line breaks are needed.
I think this requires an enhancement in the EasyFormat lib.

[SanderSpies]

LGTM

[yunxing]

Yes, we need more options in EasyFormat in order to do this. cc @pieterv who has the same requirement.

[yunxing]

Thanks!

[pieterv]

@yunxing yes i believe this requires a change to EasyFormat (see ocaml-community/easy-format#11). I did manage to implement a hack (as suggested by @jordwalke) where we insert a token in places we potental want trailing commas and then after printing from EasyFormat to string we find the instances of the token followed by a "\n" and replace with ",\n" otherwise strip it. Its working well so far although can mess with the indentation a little since you remove chars after doing all the breaking.