Enable source maps in output by exposing browserify debug option

[tmaslen]

Hi,

This is a request which I think would make a great feature to this very useful plugin.

Could you optionally expose browserify's debug option, this would allow users to enable source maps and so benefit from better error stack traces even when the output is minified.

Browserify state that you can set debug: true to enable source maps...

       --debug -d  Enable source maps that allow you to debug your files
                   separately.

On this line here: https://github.com/FidelLimited/serverless-plugin-optimize/blob/master/src/index.js#L382

If you're open to it I'd like to make this in a pull request (this might take me a bit of time).

Thanks,
Tom.

[tmaslen]

I made this feature myself and stuck a PR in for you: #115

I've plugged my version up into my build and am now benefiting from nicer stack traces.