[Feature] Allow setting the number of compilation threads when running with --compile

[zicklag]

It would be great if you could tell Koremake how many threads to use when compiling projects. Without it you have to run Koremake before manually running the command to compile the project with extra threads.

I could be ignorant to a way to do this already.

[RobDangerous]

It automatically uses as many threads as you have cores on some systems. What host and target are you talking about?

[zicklag]

I'm on Ubuntu 16.04 and I have 4 cores. It was creating only two clang++ processes as it was building.