Add support for multi-function advice

I just ran into this problem and realised Advice API could be improved to intuitively allow specifying multiple methods at once. With my commit I can now write

```javascript
this.around('read write', function (func, key, value) {
  if (this.supportsLocalStorage) {
    return func(key, value);
  } 

  return false;
});
```

so that I'll have to make this check only once for both `read` and `write`.

The existing tests pass for me with this commit. If you find it useful, I can go on to write another test for this use case.