versioning: plugins report a list of supported versions

Further progress on versioning support (Issue #266).
Bump CNI spec version to 0.3.0

plugins/main/loopback/loopback.go

@@ -18,6 +18,7 @@ import (
 	"github.com/containernetworking/cni/pkg/ns"
 	"github.com/containernetworking/cni/pkg/skel"
 	"github.com/containernetworking/cni/pkg/types"
+	"github.com/containernetworking/cni/pkg/version"
 	"github.com/vishvananda/netlink"
 )
 
@@ -67,5 +68,5 @@ func cmdDel(args *skel.CmdArgs) error {
 }
 
 func main() {
-	skel.PluginMain(cmdAdd, cmdDel)
+	skel.PluginMain(cmdAdd, cmdDel, version.Legacy)
 }