Add support for conformance groups

[dthaler]

Conformance groups are added to draft-ietf-bpf-isa, so the conformance runner should report which groups pass conformance checks.

[dthaler]

Most of this was done in PR #196.

However, the draft mentions that some groups include other groups:

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

How should one specify just 32 bit conformance, E.g., base32+atomic32+divmul32?
I believe the current answer is to use v4 with --exclude_groups base64 atomic64 divmul64
The help text should really provide an example.

Also the help text should the set of strings that are legal to use.