Searchpath

CB-5006: Install plugins using IDs and searchpath

Requires changes not yet merged into cordova-cli and cordova-plugman
CLI: https://reviews.apache.org/r/16854/
Plugman: https://reviews.apache.org/r/16739/

This will pass the plugin search path to "cordova create" as part of the extra
config parameter to be saved in config.json. "cordova plugin add" then uses
this search path when fetching plugins.