Add option to specify config path

[Zakini]

As far as I can tell, backpack currently requires the config file to be at the project root and named backpack.config.js, with no way of changing this. It'd be nice to be able to pass the path to the config file when invoking backpack, similarly to how webpack handles it.

Something along the lines of this: backpack --config /path/to/config.js

[lubien]

I could work on this.