Implements --update-section which is currently supported for ELF for Mach-O as well
From what I can gather about Mach-O, it seems like maybe updateSection could just be removeSection + addSection, but I wasn't sure. Maybe someone with more Mach-O experience could say for sure.
we probably don't need 2 cases here (specified / unspecified segment name),
std::tie(SectionName, FileName) = Flag.split('=');