Support custom toolsets

[lochnessdragon]

I'm trying to use premake-ninja with premake-emscripten, which should function as a drop-in replacement for gcc/clang. Unfortunately, the if branches in compilation_rules() only allow msvc, gcc, or clang. I've modified a version of premake-ninja to also allow emcc, but maybe there should be an API to implement the per-compiler commands in a more generic basis?

[tritao]

Hey @lochnessdragon, currently looking to use the emscripten module with this ninja generator too.

Just wondering if you could send a PR or share the changes you mentioned.