Enable pass on non-linux systems

There is nothing linux specific about pass. In particular, I use it in
openbsd.

Signed-off-by: Rafael Ávila de Espíndola <rafael@espindo.la>

pass/cmd/main.go

@@ -0,0 +1,10 @@
+package main
+
+import (
+	"github.com/docker/docker-credential-helpers/credentials"
+	"github.com/docker/docker-credential-helpers/pass"
+)
+
+func main() {
+	credentials.Serve(pass.Pass{})
+}