need to be able to specify rollup options/plugins

I was trying to find a good way to allow this behavior and submit a PR, but nothing elegant was jumping at me. There are two main problems:
- compiled's configuration is in JSON
- Rollup's configuration file is executed JavaScript

So, I can't use plugin functions in `package.json` obviously, and I can't specify my plugins for Rollup in any manner other than in a `.js` file.

Potential solutions:
- support a `.js` config file in addition to `config` prop of `package.json`
- allow a new option which specifies a path to a Rollup config file (I'm not sure how well this would work with multiple entry points, or if it would be mutually exclusive with `files` property)