Add support for header extensions

[jonpas]

• Close Add support for header extensions #64 Add support for PboPrefix.txt #43
• Argument for header extensions (any in addition to prefix)
• Write argument header extensions to PBO header on build
• Read all header extensions (any in addition to prefix) from PBO header on unpack and inspect
• Fix cat when multiple header extensions are present
• Add argument header extension validation (required =)
• Allow overwriting of prefix header extension read from $PBOPREFIX$ by argument header extension

[jonpas]

Done. String operations could probably be improved, am open to suggestions.

Should we add a note when prefix is being overwritten? If so, is there a helper available like for error and warning?

[pixlcrashr]

@jonpas @KoffeinFlummi Source built and run successful on Debian 9 machine.