Add principal variation

[znxftw]

Store pv line instead of just bestmove

This gets easier if #33 is played first as we can just iterate through the TT to get the PV line once the search completes - no need add extra logic to the search to maintain the PV

[znxftw]

I've tried out with TT - it gives a line but usually not deep enough
Maybe because of overwriting? I'll dig into it a bit

[znxftw]

This is currently in place but I'll keep this open - maybe better ways to extract PV lines somehow.