Missing support for the `__generator` helper

[dcherman]

Running the code on this example (https://www.typescriptlang.org/play/index.html#src=(async%20function%20()%20%7B%0D%0A%0D%0A%7D)%3B) through this rollup plugin seems to not emit the __generator helper.

[dmitry-korolev]

You should upvote #75
It's waiting for approval for a month already

[PaulBGD]

Closing issues that I've integrated into #91, will be merged into master soon.