Added a cl tool to start mycroft to listen: mycroft-listen (#2460)

Add a cl tool to start mycroft to listen: mycroft-listen

Alternative triggers. #1357 - Added feature to activate via command line
This can be triggered in most desktop enviroments via short cuts


====  Documentation Notes ====
Activates the mycroft listening feature.

 On branch feature/mycroft-listen_cli_tool
 Changes to be committed:
	new file:   mycroft-listen

bin/mycroft-listen

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+
+# Copyright 2020 Mycroft AI 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.
+
+SOURCE="${BASH_SOURCE[0]}"
+cd -P "$( dirname "$SOURCE" )"
+DIR="$( pwd )"
+
+# Enter the Mycroft venv
+source "$DIR/../venv-activate.sh" -q
+
+# Send a message to be spoken
+python -m mycroft.messagebus.send "mycroft.mic.listen"