Add ping command

lib/npm.js

@@ -141,6 +141,7 @@ var commandCache = {}
               , "prefix"
               , "bin"
               , "whoami"
+              , "ping"
 
               , "test"
               , "stop"

lib/ping.js

@@ -0,0 +1,19 @@
+
+var npm = require("./npm.js")
+
+module.exports = ping
+
+ping.usage = "npm ping\nping registry"
+
+function ping (args, silent, cb) {
+  if (typeof cb !== "function") {
+    cb = silent
+    silent = false
+  }
+  var registry = npm.config.get("registry")
+  if (!registry) return cb(new Error("no default registry set"))
+
+  npm.registry.ping(registry, function (er, pong) {
+    cb(er, er ? null : pong)
+  })
+}