Dedupe (license) comments

[evan-scott-zocdoc]

It'd be nice if this plugin automatically deduped the license comments it collects. For instance, Facebook includes a license header at the top of most files which becomes very cluttery, since they're all essentially the same.

[alexander-akait]

Partial fixed in https://github.com/webpack-contrib/uglifyjs-webpack-plugin/pull/383 (for extracting comments)