Add the ability to define an install path per dependency

[kodie]

This would probably only be able to be set via the JSON file, I'm thinking something like this:

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

This is kind of related to #2

I think also if you used the install command to install a specific package and set both the --install-path and --save options and the install path is different than the default one, it would set the install path in the JSON file.