Operators for number values

[DSteve595]

I've always wanted to be able to use values that are math operations, usually addition. For example:

`android:paddingBottom="@dimen/fabSize + 8dp"

That's based on regular Android XML syntax, not the syntax this project uses, but something that allows for operators to be used on dimen values would be really awesome.

[cypressious]

If something made me use this, it would be this. To reduce complexity, you could mandate that resources with dimensions never have qualifiers so that you can implement it by copying all the modifiers of the references resource.