Allow some options to be set via the JSON file

[kodie]

I think this would only be applicable for the --install-path command.

I'm thinking something like:

{
  "settings": {
    "gpm": {
      "install-path": "content/gpm_modules"
    }
  },
  "dependencies": {
     "derek-watson/jsuri": "1.3.1",
     "eduardoboucas/include-media": "1.4.10"
  }
}

or something like that. Options could be of course overwritten via the command line and paths would be relative to the JSON file.

[kodie]

This was added in fd31a50