Add rate limiting to the image processor.

lib/cli.js

@@ -512,6 +512,9 @@ program
   .option('-r, --storage-url <url>'
     , 'URL to storage client'
     , String)
+  .option('-c, --concurrency <num>'
+    , 'maximum number of simultaneous transformations'
+    , Number)
   .option('--cache-dir <dir>'
     , 'where to cache images'
     , String
@@ -525,6 +528,7 @@ program
       port: options.port
     , storageUrl: options.storageUrl
     , cacheDir: options.cacheDir
+    , concurrency: options.concurrency || os.cpus().length
     })
   })