Add npm cache based downloading to lazy_load

lazy_load was downloading platforms files from github using a url like this
git-wip-us.apache.org/repos/asf?p=cordova-ios.git;a=snapshot;h=3.5.0;sf=tgz

This change adds the possibility to donwload platform file from npm registry
E.g. cordova-ios is published here https://www.npmjs.org/package/cordova-ios

By default the old way is still used, to use npm add --usenpm flag to cordova
cli or opts.usenpm=true in opts parameter of the cordova.raw.platform()
function.