Add support for PEP 525 -- Asynchronous Generators

[dbrattli]

Mypy (0.4.6) gives error: 'yield' in async function, even when specifying Python version as 3.6.

[ambv]

I can confirm this is a problem with master, too, ran with --fast-parser --python=3.6.

[JelleZijlstra]

I submitted pull requests to typing, typeshed, and CPython for some of the supporting infrastructure (python/typing#346, python/typeshed#815, http://bugs.python.org/issue29198) and I started working on implementing this in mypy, but it may take a little while longer for me to finish the patch.

[brettcannon]

Due to this not making it into 0.501, it might be good to weaken the wording on the "supports Python 3.6 features by default" on the site as I read that to mean all features instead of some features.