refactor: to build using rollup

[tdeekens]

This is a suggestion on how bundling with rollup would work and how it might ease things a little.

1. Updates .gitignore to contain more sane defaults
2. Adds rollup as a build tool and a rollup.config.js
3. Updates npm run-scripts to reflect rollup
4. Cleans up the tools to not contain the build.js anymore
5. Adds module and main entries to package.json
6. Adds run-p so that all builds with targets run in isolation and parallel

I added an output plugin to rollup as this project also seemed to have that before. It will look like this:

Closes #34

[tdeekens]

Is there any chance this could land in the main code base or should I close this PR?

[mjackson]

This is definitely on my TODO list, just hadn't taken a look yet!

In any case, we'll probably do the exact same build script and configuration we're using in React Router, just to keep things consistent between projects. But thank you for the PR!