Add custom fit labels to vp-comparefits
Created by: voisey
Aims to close https://github.com/NNPDF/nnpdf/issues/643.
This should avoid the confusion people always seem to have about which fit is which in comparison reports. Example report: https://vp.nnpdf.science/oqROkiNnRCe07mZtAv6mUw==/