(Closed via #918) The last item of dicts can end in a comma

[OAGr]

Right now, dicts like the following don't work:

x = {
  a: 1,
  b: 2,
}

The last item must not have a comma. This makes it more quickly to quickly modify dicts.

It could be nice to allow for trailing commas. I'm not sure what sacrifices this would cause.

[umuro]

This is an easy parser fix. I don't think there is any ambiguity

[OAGr]

Great, thanks!

[quinn-dougherty]

Closed: #918