Add support for loading and saving js files.

[yoneal]

Javascript files must be in CommonJS module format, solving #96

This is just to start the ball rolling. If it is more appropriate to turn this into a separate plugin instead, just let me know!

[indexzero]

This is a really neat idea! The world of webpack.config.js and other JavaScript config files has become much larger since this project was written. Evaluating JavaScript even in a new vm context is more dangerous than JSON, so would like to see this be an opt-in feature.

If you have time to make that change and add tests we'll get it landed.

... thanks again!

[Alexsey]

Probably you should remove console.log

[dlong500]

Any movement on this? It would be really nice to have the same type of flexibility as other config file systems. I get the theoretical argument that JS is more dangerous than JSON. However, given the context of numerous build tools and other modules high adoption rate of JS config files it seems like the security issue is a bit overstated. And besides, if someone can hack one of your config files they can likely hack any file in your entire app.