Transformers: Better gulp plugin integration

[LukasHechenberger]

It should be easier to call gulp plugins from transformers. Currently gulp plugins can be called like:

import gulpPlugin from 'somewhere';
import { Transformer } from 'atscm'

export default CustomTransformer extends Transformer {

  ...

  transformFromFilesystem(file, enc, callback) {
    gulpPlugin({ /* options */ })
      .on('error', err => callback(err))
      .on('data', data => callback(null, data)
      .write(file);
  }

  ...

}

[LukasHechenberger]

Implemented in #49, shipped with release v0.3.0