Adding plugins should support --variables for plugins that require them

Currently the phonegap cli has no support for variables when adding plugins.

http://docs.phonegap.com/en/3.1.0/plugin_ref_spec.md.html#Plugin%20Specification

After a quick chat  in the phonegap IRC room a while back I was told that plugman shouldn't be required. I have ran into the problem where I want to be able to setup the Facebook plugin and have the ID and App name a required parameter. It would be nice to be able to support this from the phonegap cli.

I would imagine it working like the plugman version where it supports `--variable VAR=VALUE`

Referencing an issue as a reminder on another project here https://github.com/logankoester/grunt-phonegap/pull/6