Establish a go.mod (and go.sum) file such that this library can be
handled as a Go module. This has the added benefit of now being
useful outside of gopath.
I do not believe we need to release to provide/use module support but
we should possibly start considering creating v0 tags soonish.
See also: https://blog.golang.org/publishing-go-modules
Signed-off-by: John Mulligan <jmulligan@redhat.com>
