Add option to specify config path

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```

Start at the backpack command-line entry point and trace how it locates backpack.config.js. Check the existing command help and configuration-loading behavior, then verify that a --config path is accepted and that the specified file is used instead of the project-root default.

Written by the indexing model from the issue text.