Add support for inlining external examples

This probably shouldn't be scoped to just @example, but that's my specific use case.

Basically, I've written examples in my source code comments, but as a result, I can't unit test them, lint them, etc, so they tend to have typos and whatnot.

src/constructable.js

``` JavaScript
/**
* @example
* <%= examples/constructable.js %>
*/
class Constructable {

}
```

examples/constructable.js

``` JavaScript
var constructed = new Constructable()
assert.instanceOf(constructed, Constructable);
```

which would produce the same output as

``` JavaScript
/**
* @example
* var constructed = new Constructable()
* assert.instanceOf(constructed, Constructable);
*/
class Constructable {

}
```

Right now, my plan is to solve this with a mix of documentationjs and grunt templating, but it would be great to see something like this natively in documentationjs (or via a plugin, if #70 gets traction).