Variant specific properties file

Add support for variant specific properties. Right now, the plugin supports buildType _OR_ flavor specific properties.

For example, let's say we have build types **release** and **debug** and we have flavors **paid** and **free**. Accept `paidRelease.properties`, `paidDebug.properties` , `freeRelease.properties` and `freeDebugProperties` instead of only `release.properties`, `debug.properties`, `free.properties` and `paid.properties`.