add git commit hash to version in form X.Y.Z.dev-gitgitsha-indexclean

[oplatek]

Hi there!

I added git SHA to the version in setup.py in the format X.Y.Z.dev-SHA-dirty or X.Y.Z.dev-SHA-clean
The clean option is added when the git index is clean (on all releases).

I find it particularly useful if Lhotse is installed via pip install git+https://github.com/lhotse-speech/lhotse

$ pip list | grep lhotse   # see the second column for the version example
lhotse                        0.6.0.dev-2840393-clean /home/oplatek/code/lhotse

[pzelasko]

Thanks, looks great!