Command line switch to specify slug.json

I found during my workflow that it is sometimes useful to have different versions of slug.json in the same project in order to be able to for example include different libs depending on the platform. In my case, I find this issue when working on cross-platform phonegap projects where often plugins as well as the phonegap file itself, have different versions depending on the platform.

I created an optional command line switch -s or --slug in which you can specify the path to the slug.json that you want when using hem. Since this is an optional switch, if you don't specify anything the behaviour is normal. i.e. we look for ./slug.json

Example:

hem watch -s slug-android.json
hem server -s slug-ios.json