Fix custom option parsing to accept quoted strings

This is a backward compatible change to prepare for gaudi/Gaudi!577 (merged), similar to Rec!2139 (merged) and LHCb!2659 (merged).

In particular, this fixes the failing Moore test in https://lhcb-nightlies.web.cern.ch/nightly/compare/lhcb-master-mr/1087/lhcb-master-ref/760/ (gaudi/Gaudi!577 (comment 3631662))