config command to modify config.xml preferences via command line

This PR allows the user to modify the preference tags on the config.xml file of the project using the command line, instead of requiring manual edition of the file. 
This is not limited to the existing preferences, it also allows to create new custom ones.
It will be very handy to automatic process to change the default orientation or force fullscreen or change the color of the status bar (ios) on a simple step.
It also allows to introduce new access directives, also handy to avoid using the * by default.

Sample of usage:
phonegap config ./my-app -p orientation=landscape -a "http://myhost.com/*"