Add decode subcommand

Accepts decimal or 0x-hex GID input. gid=0 is rendered as the
literal 'empty'; non-zero values print the three components in a
parse-stable space-separated key=value form.

src/cli.js

@@ -31,6 +31,7 @@ function run(argv, { out, err }) {
   }
   const handlers = {
     encode: require('./commands/encode'),
+    decode: require('./commands/decode'),
   };
   const handler = handlers[head];
   if (!handler) {