Allow plugins to set ext.multiarch to enable multiple APK building

The xwalk webView need build multiple apks by default after install
cordova-crosswalk-engine plugin, we can set ext.multiarch=true to open
the flag in plugin, it don't necessary set system environment BUILD_MULTIPLE_APKS
manually.

This closes #141
Merge branch 'master' into 4.0.x (Plugin set multiarach)