Add option to pass argument to verilator's make. Useful to speed up …

[benreynwar]

…compilation.

I'm running verilator on a large design and the compilation was taking a long time. I wanted to able to call 'make -j 16' to speed things up, but there wasn't an option to pass arguments to verilator's make command.

I just called the argument 'more_compile_args' for lack of any better ideas, and in case some other simulator could also make use of it.

On a side note, if anyone has any suggestions for ways I can make verilator compilation less slow for large designs I'd love to hear them!

[themperek]

If anything I would change this to make_args and add this only to the verilator class.

[dramoz]

Please close, new PR 232 created with updated master and proposed re-work changes.

[themperek]

Close in favour of https://github.com/themperek/cocotb-test/pull/238