Add multi-thread support

[bezirg]

This is a feature request.

For large projects, checking in multiple threads could parallelize the I/O and processing time, hopefully yielding better performance.

Also, it should be worht to provide an option flag to control the number of threads used.

[mstruebing]

I actually remember back when I was implementing this tool in its first iteration I tried to run it with go routines in multiple threads but the result was actually slower than a single thread.

Maybe that changed with all the changes in between, and I'm not entirely sure but I believe I've had an HDD back then, no SSD. So it may be worth trying and benchmarking it first before building a solid solution.