Add rnpm like plugin support to Haul

[satya164]

trafficstars

It'd be great to be able to add additional commands to haul like in rnpm. We could have a separate plugin which provides a single command to automate publishing to codepush etc.

[grabbou]

Yeah, it doesn't take a lot of lines, can backport from react-native.

[grabbou]

Let's wait for someone to raise this use-case, for now, it's icebox.

[grabbou]

CC: @zamotany since you mentioned some plugins proposal.