Support reading of <feature> param values other than "onload" and "x-package" keys

There is no component for this, this relates to config.xml.
For the proposed solution, there are no changes needed to the config.xml specification, since the feature tag already supports param child tags.

I'm only speaking for iOS, not sure about the other platforms.
The problem is, there is no access to these additional values in plugin code. The plugin has to re-read the config.xml to access this.

What are the alternatives in config.xml/plugin.xml?

1. preference tag on the plugin level
2. preference tag on the platform level

The problem with either of these two approaches is that if another plugin has the same key, it will clobber your preference. The ugly solution to that is, to namespace your key with your feature name as a prefix.

Proposed solutions:

1. Create a plugin in cordova-labs to read these extra param values without touching the core code.
2. Modify the core code to read these extra param values, and expose them to the user