Add `--onEachCompilationStarted` option

[zanminkian]

I want to run rm -rf dist before each compilation. --onCompilationStarted seems only work at the beginning (initial), not each compilation.

--onCompilationComplete will work after every compilation, so we need an option to work before every compilation. I think it make sense.

[gilamran]

The watch option is implemented by typescript itself, and not tsc-watch. That's why I can not hook into the "before" compilation...