Allow for source map mode selection

[kpdecker]

Running into issues within my workflow with cheap-module-source-map and would be awesome to be able to configure this through some mechanism. (bonus: anything in the webpack config)

[thangngoc89]

There is a PR for it already. Stay tuned. #56

[rkostrzewski]

In 1.3.0 you can do:

export default (config, env, helpers) => {
  config.devtool = 'your-own'
}