Generate a default arg_name for fields one isn't specified

[erickt]

As suggested in https://github.com/google/argh/pull/112#discussion_r764403695, @richkadel suggested that if a field doesn't have an arg_name specified, we could generate one automatically for it to make it prettier for help messages or redactions. We should consider doing:

• lower case the field name
• convert _ to -
• remove leading _