getopt style option parsing for Go
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.
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).
Hopefully getopt will now work with go modules.