Add --numeric-version flag.

The json output flag just added is meant for other tools. Adding `--numeric-version`, which is also meant for tools, in the same release seems reasonable. This change allows HLint to preserve the numeric-version behaviour across different releases of cmdargs. 

I hacked the flag into main mode. I'm not sure if it should go elsewhere--it's definitely a common flag. Setting the group name for the flag alters the group name for the entire mode rather than putting the numeric-version flag in the common section. 

I still have the TODO for cmdargs support on my list, but haven't gotten around to that yet.