Update help text

[dthaler]

Expand help text to include examples, and valid values of options

Also update conformance groups list to include other groups that the spec says to, i.e.:

base64: includes base32, plus instructions explicitly noted as being in the base64 conformance group
atomic64: includes atomic32, plus 64-bit atomic operation instructions
divmul64: includes divmul32, plus 64-bit division, multiplication, and modulo instructions

[coveralls]

coverage: 95.439% (+0.2%) from 95.212%
when pulling 517f9a2 on dthaler:help
into 7d59494 on Alan-Jowett:main.