Would appreciate an ES6-only build

[pkese]

Congrats - I find this to be the best lightweight implementation of a Promise based Xhr.

However I'd like to use it in a project, where I'm having core-js for all ES6 browser polyfills, so I find it quite redundant having to pull in bluebird and extend libraries just for this project.

Is there any chance of having the build process producing two versions of the library: one like now and one where it is assumed that Promise already exists and extend is Object.assign.

Sorry, for not being familiar with coffeescript, otherwise I'd write a PR myself.

Thanks