Add --k12 and --m14 alias switches (#6)

This adds shortened aliases --k12 and --m14 for the existing switches --kangarootwelve and --marsupilamifourteen, intended to improve ease-of-use.

Implement checksum verification (#4)

This implements the -c / --check switch to verify checksums previously generated by ksum, similar in behaviour to other checksum programs such as sha256sum.

A test suite is also added to test the behaviour of ksum, including tests against various test vectors.

The --warn, --quiet, --strict switches are also implemented (only relevant when verifying checksums)