Allow passing additional dex parameters to create multi-dex builds

Sorry, this is very raw change currently, I'm struggling with dex+proguard cache, feels messy (esp with scala.jar + proguard assumptions) but at least I made it work currently, so now it can be polished to a fully working solution, without killing proguard-ability.

I'm making this pull request early to hear from you about this change, any comments and help would be appreciated.

Checkout https://github.com/casidiablo/multidex for more details
rename to `dexMulti`