Add pruning step to skeletonization

This requires an updated Fiji, as detailed in this mailing list thread:
https://list.nih.gov/cgi-bin/wa.exe?A1=ind1308&L=IMAGEJ#41
https://list.nih.gov/cgi-bin/wa.exe?A2=ind1308&L=IMAGEJ&F=&S=&P=36891