Added optional asyncawait option on top of esnext.

Seems to work, passes the test suite, works well with arrow function and classes. 

use the following options:
```
  "esnext": true,
  "asyncawait": true
```

To try it:
```
npm install -g -f https://github.com/sebv/jshint/archive/esnextnext.tar.gz
```