Pass through annos, support for black box verilog implementations (#93)

* black box annotations working but verilator does not know to open the right file

* vcs and verilator backends now have annotated black box verilog implementations delivered to the build directory
this is done by a firrtl transform
Test that this adding is done correctly

* removed dangling transform from VerilatorCppHarness, transform is now added in firrtl backend
made tests safely