Add admin profiling command (#2557)

Implement profiling command to start & download profiling data of
a standalone or all nodes of a cluster.

```
1. Start CPU profiling
   $ mc admin profiling start --type "cpu" myminio/

2. Download latest profiling data under save under profiling.zip
   $ mc admin profiling stop myminio/
```

cmd/admin-profiling.go

@@ -0,0 +1,40 @@
+/*
+ * Minio Client (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import (
+	"github.com/minio/cli"
+)
+
+var adminProfilingCmd = cli.Command{
+	Name:   "profiling",
+	Usage:  "Generate profiling data for debugging purposes",
+	Action: mainAdminProfiling,
+	Before: setGlobalsFromContext,
+	Flags:  globalFlags,
+	Subcommands: []cli.Command{
+		adminProfilingStartCmd,
+		adminProfilingStopCmd,
+	},
+	HideHelpCommand: true,
+}
+
+// mainAdminProfiling is the handle for "mc admin profiling" command.
+func mainAdminProfiling(ctx *cli.Context) error {
+	cli.ShowCommandHelp(ctx, ctx.Args().First())
+	return nil
+}