allow to add a plugin from drive different from c:

[Athorcis]

This commit alllow to add plugin from a drive different from c: when using bash under Windows.
Therefore the following command now work:

cordova plugin add /d/home/cirkwi/mobile/plugins/cordova-plugin-assets

[agrieve]

Thanks for the patch! Looks like in a couple of other files, we do:
./cordova/config.js: else if (uri.protocol && uri.protocol[1] ==':') return uri.href;
./cordova/lazy_load.js: isUri = uri.protocol && uri.protocol[1] != ':';

So, I've used that check instead of your PR to make the fix.
The relevant JIRA issue is https://issues.apache.org/jira/browse/CB-7610