Structure of Arrays Components and Custom Allocator Support

I've been looking at entityx and it seems like a great library, thanks for makings such a great library :+1: .

Is there any plan/desire to add support for defining components as a struct of arrays (as opposed to making them arrays of structs as they are now)? I'm thinking something along the lines of what's described in [this BitSquid post](http://bitsquid.blogspot.com/2014/09/building-data-oriented-entity-system.html) and the one before it.

Hand-in-hand with that goes custom allocator support. I know entityx already uses memory pools to keep cache coherency, but is there any way to provide your own allocation scheme to be used? I'll admit that I haven't looked too much into whether this option is already available, but it would be necessary to allow struct-of-array type components.

Thanks again for your hard work on entityx.