Trying to make go modules somewhat happy.

This release is to keep go modules happy.

Apparently you cannot have a v2 sub-directory unless you have a v2.x.x. tag. Unfortunately other things see this tag and claim getopt.v2 exists along with getopt/v2.

Add Mandatory and Radio Group options

This release introduces mandatory options and radio group options.

Any mandatory option not seen during parsing will cause an error.

At most one option from a radio group may be specified (mutually exclusive options).

go.mod release

Hopefully getopt will now work with go modules.