Add webpack@4 support.

[sanex3339]

Webpack@4 just released, so it will nice to have it support (it backward compatible but it's nice to support new API).

https://medium.com/webpack/webpack-4-migration-guide-for-plugins-loaders-20a79b927202

• plugin method is backward-compatible
• plugins should use Compiler.hooks.xxx.tap(, fn) now

Other breaking changes related to plugins: https://github.com/webpack/webpack/releases/tag/v4.0.0

[jrparish]

Published v1.2.0 with support for webpack 4. Let me know if there are any issues.