Add (minimal) ProForma support in mokapot.picked_protein.strip_peptides

[RalfG]

When adding a ProForma sequence with a terminal modification (e.g., [+304.207].AINEAYK[+304.207]EDYHK[+304.207]), the strip_peptide function does not remove the dash that indicates the N-terminal modification. This breaks peptide-protein mapping downstream (for instance in #114, I think). Can support for ProForma also be added? Thanks!