Add support for redirecting feature configs to file

This is what I was asking about adding here:

http://karaf.922171.n3.nabble.com/Problems-With-Factory-Configurations-In-Karaf-3-0-1-td4033921i20.html

By being able to redirect configurations defined in feature files to file (rather than direct to configuration admin) makes these configurations much easier to manage. I have tested it myself and everything appears to work as expected. The default behaviour will be to work as before (i.e. the feature configs go direct to config admin).