Added optional asyncawait support on top of esnext.

Seems to work, passes the test suite, works well with arrow function and classes. 

Configure like in the following:
```
"esnext": true
"experimental": ["asyncawait"]   or "experimental": ["asyncawait", "asyncreqawait"]
```

To try it:
```
npm install -g -f https://github.com/sebv/jshint/archive/esnextnext.tar.gz
```

EDIT updated options.
> I agree it'll be better to pass an object.

Yeah. Not in this patch, but that has to be addressed.

> The function parameter list issue is a preexisting issue which need to be tackled by the core team

You're right. Sorry for dumping that baggage on you.