Polyfilled Node version of Rollup

[jeroenptrs]

Since jspm support for Deno is way in the future for now, this will probably have to be done manually or through the Dreg project (denofn/dreg).

Ultimately this would allow denopack's CLI to become more of a wrapper for functionality proposed in #8 rather than a Denoified version of the Rollup CLI.

Since the future of dreg is still undecided, I'm opening an issue here as well.