Support for async functions

[trusktr]

It would be useful to transpile async functions, they're a valid part of the language.

See here for previous discussion: https://gitlab.com/Rich-Harris/buble/issues/71

The fast-async package provides a leaner/faster result than regenerator (let's not use regenerator), and I think it can be further optimized because when I convert async functions to Promise-based equivalents manually, they always come out with the leanest result. The new Promise.finally will help with this.

This may require a Promise polyfill to be supplied by the user, and that should be documented.

[IvanSanchez]

See #12

[shirotech]

See #125 also, hope it gets approved :)