Type links

It would be nice to be able to link to a type that is documented elsewhere on the web. For example you might have a function like:

```
@function xhr
@param {Object} options
@return {Promise} a promise that will resolve when the xhr is complete.
```

In which case you would want Promise to link to the MDN documentation. Perhaps "@typelink" could do this:

```
@typelink Promise https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
```