Support async functions in export declarations

I noticed that 6to5 is able to transpile
```
export async function foo(){}
```
to

```
"use strict";

exports.foo = foo;
function foo() {
  return regeneratorRuntime.async(function foo$(_context) {
    while (true) switch (_context.prev = _context.next) {
      case 0:
      case "end": return _context.stop();
    }
  }, null, this);
}
```

but acorn-6to5 is unable to parse it. Why would that be?

Given that the [proposal](https://github.com/lukehoban/ecmascript-asyncawait#syntax) extends *Declaration* to include *AsyncFunctionDeclaration*, it seems the parser should support this. I'm not that familiar with acorn (or parsers in general), so I don't know if there is a better way to do this.

I added a new test case, however two other tests unrelated to this diff are currently failing.