Update to properly handle MultiCompiler instances

All of the compiler setup and plugin operations were assuming a singular Compiler instance vs. MultiCompiler constructs. This changes the behavior to allow for iteration over multiple compilers where necessary.

Fixes #31
I admit that I did not get to the point of actually looking at test exec with multiple, just handing single entry array configs and I can see how overwrites could be a problem here.

I'm not 100% clear on what you'd like to see here or how to get there. Rather than concat files, it seems like multiple different tests should be run, one for each environment as it seems that the reason for having multiple builds is to have distinct execution targets (or at least that is my particular reason).

Glad to keep researching.