Instead of being hard/fast about the `engine` being one of the smith instances, can we allow this to be a string as well? (e.g. `engine = 'pngsmith'`). Then, we could load it via `require`.

```
engine = params.engine,

if (typeof engine === 'string' ) {
  engine = require('engine');
}
```

It would make for a cleaner transition and allow for not needing to load the engine when we aren't running the `grunt-spritesmith` task.