require config file to allow for json or js files

Allowing for the config file for migrations to be a **json** or **js** file gives some nice flexibility to the user.  Node added support for requiring json files a couple years ago in **0.5.2**.  My PR would probably mean having to bump up `engines.node` to at least that to be safe.  If maintaining support for **0.4.x** is desired, I'm happy to change what I have to branch the logic for json/js files if that's preferred.