Add a custom fetch function to handle things like auth, etc...

[mghignet]

Hello,

I forked the project to meet one of my needs (there should be some other ones later) which is to be able to pass custom parameters, like authentication, custom headers or to pre-process the user input before fetching. That's what I have added to this pull request, along with some modification to the .gitignore and a dependency added to package.json, because babel-cli was used in the NPM scripts. I also updated the README.

Feel free to comment and merge if you like.

Thanks a lot for your library and merry Christmas!