Move cobra.MousetrapHelpText to hugo_windows.go

Fix build error due to Cobra API change, see
spf13/cobra#195: Compile mousetrap only on Windows

commands/hugo_windows.go

@@ -0,0 +1,27 @@
+// Copyright 2015 The Hugo Authors. All rights reserved.
+//
+// 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 commands
+
+import "github.com/spf13/cobra"
+
+func init() {
+	// This message to show to Windows users if Hugo is opened from explorer.exe
+	cobra.MousetrapHelpText = `
+
+  Hugo is a command-line tool for generating static website.
+
+  You need to open cmd.exe and run Hugo from there.
+  
+  Visit http://gohugo.io/ for more information.`
+}