Support webpack 4 and plugin maintenance / dev fixes

[chrisblossom]

This PR:

• Adds support for Webpack 4, while preserving support for the old plugin system.
• Moves over to the recommendedbabel-preset-env and requires node 4+ (might want to require node 6 lts+ as node 4 is no longer support starting the end of this month).
• Upgrades packages to latest versions and fixes all lint issues.
• Fixes sandbox (was not previously working) and upgrades sandbox config to be compatible with Webpack 4.

I verified Webpack 3 still works as expected.

Closes #50, #32, #52.

I know it isn't normal to have one PR with so many changes, but I was running into several things that was stopping development. For instance, there was a bug with Eslint v3 that was causing a lint error saying type UserOptionsType = { was not defined. Also, the sandbox config did not work at all.

If you want this PR split out into separate ones, or want something changed or removed, let me know.

[gajus]

Thank you.