Add encode subcommand

Wraps gid.encodeGid, prints the packed-u32 as decimal on stdout,
emits usage on stderr on bad arity, and a labelled error on
out-of-range or non-integer input.

src/cli.js

@@ -30,7 +30,7 @@ function run(argv, { out, err }) {
     return 0;
   }
   const handlers = {
-    // populated in subsequent tasks
+    encode: require('./commands/encode'),
   };
   const handler = handlers[head];
   if (!handler) {