(Partially) fix #30 (#31)

* (Partially) fix #30

With this change (and a renamed parse-opcodes) it's possible to as a
Python module without having to patch the repo.  Example:

  from parse_opcodes import parse_inputs

  if __name__ == "__main__":
      (namelist, pseudos, mask, match, arguments) = parse_inputs(["opcodes", "opcodes-rvc"])

* Fix #30: Rename parse-opcode to parse_opcode to enable module use