add ability to control position of injected files

It would be nice if developer can control way how baggage-loader adds injected files:

• before source (prepend) return inject + source;;
• after source (append) return source + inject;;

In some cases need to inject baggage files after the source, because these files depend on source.

{
    test: /angular-material\/modules\/js\/.*\.js$/,
    loader: 'baggage?[file].css&[file]-default-theme.css&default-theme.js&[file]-default-theme.js'
}

require('angular-material/modules/js/core/core');

default-theme.js or [file]-default-theme.js should be added after angular-material/modules/js/core/core.js

https://github.com/deepsweet/baggage-loader/issues/11