Feature: Explicit permissions list in AndroidManifest.xml

In #53, @olanod requested the ability to set any property in `AndroidManifest.xml` through the `grunt-phonegap` config object, giving explicit permissions as a use case example.

I'm hesitant to go that far, but that particular use case is valid and one that I have encountered as well, so I have implemented the ability to set the permissions list manually in this way.

See [permissions.md](https://github.com/logankoester/grunt-phonegap/blob/c8bcdd9f0cc4ab105046371b86c3ae942777ca7e/docs/features/permissions.md) for documentation.