Option to disable constantFoldingPlugin in production builds

[olivergeorge]

This is a feature request.

Constant folding breaks some already minified code. In order to do a release build with minified code its necessary to patch the worker.js file to comment out plugins.push([constantFoldingPlugin, opts]);.

#291 describes the problem this feature would resolve. This is still a problem as of 0.62.2.

Adding an option here to disable constantFoldingPlugin would provide a simple configuration based solution.