Add an option to show estimated pitch on the spectrum.

[celeste-sinead]

This uses a simple 3-Harmonic Product Spectrum. It works well when tested on my own voice (whereas Fmax often makes an octave error with the first overtone).

[tlecomte]

Thank you very much @celeste-sinead!

I've tested it and it works well indeed. I've noticed a few things that are worth adjusting in the implementation, but I'll merge anyway and try to address those in a following commit.

Thanks again for your contribution.