[CB-1108] Convert config.xml from <plugin> to <feature>

This unifies the config.xml used by iOS and Android. Now <feature> tags can be used
to add plugins. The changes are non-intrusive so the older versions of the config.xml
can still be used after this change. The value of a param named "android-package"
determines the class for the plugin.
Deprecate <log> in favour of <preference name="loglevel">
[CB-1108] Make onload=true settable as a <feature> <param>

Plus some simplification of code.
[CB-1108] Update config.xml template's list of available <preference>s