WIP: feat: transpile object spread

Merged username-removed-498219 requested to merge dariocravero/buble:object-spread into master

I know this isn't a standard feature yet but it is a pretty handy one that helps write very succinct code. I understand if it's not in the spirit of buble to support it. It doesn't really add a big overhead, so I figured I'd submit it for inspection and let the community decide.

I flagged it as WIP because there's one test that isn't working and maybe someone could help, here's the offending test:


  1) buble object-rest-spread.js transpiles object rest spread nested:

      AssertionError: 'var c = Object.assign({}, a, {b: 1}, {dd: Object.assign({}, d, {f: 1}}), {e: e});' == 'var c = Object.assign({}, a, {b: 1}, {dd: Object.assign({}, d, {f: 1})}, {e: e});'
      + expected - actual

      -var c = Object.assign({}, a, {b: 1}, {dd: Object.assign({}, d, {f: 1}}), {e: e});
      +var c = Object.assign({}, a, {b: 1}, {dd: Object.assign({}, d, {f: 1})}, {e: e});

Thanks! :)