Add inspect subcommand: read-only stats for v2 map files

Reads the map JSON, optionally loads --atlas-dir paths for name
resolution, and prints the inspector report to stdout. Missing
file, bad JSON, and missing --atlas-dir paths all produce
labelled stderr errors with exit 1.

src/cli.js

@@ -33,6 +33,7 @@ function run(argv, { out, err }) {
     encode: require('./commands/encode'),
     decode: require('./commands/decode'),
     build: require('./commands/build'),
+    inspect: require('./commands/inspect'),
   };
   const handler = handlers[head];
   if (!handler) {