Allow api key to be set from environment variable

In dockerised environments, secrets are usually set
from environment variables to allow them to be
overlayed securely in cases where configuration
is given by docker-compose.

Signed-off-by: Christopher Obbard <chris@64studio.com>

autodoist.py

@@ -1,6 +1,7 @@
 #!/usr/bin/python3
 
 from todoist.api import TodoistAPI
+import os
 import sys
 import time
 import requests
@@ -902,7 +903,7 @@ def main():
     parser = argparse.ArgumentParser(
         formatter_class=make_wide(argparse.HelpFormatter, w=120, h=60))
     parser.add_argument('-a', '--api_key',
-                        help='takes your Todoist API Key.', type=str)
+                        help='takes your Todoist API Key.', default=os.environ.get('TODOIST_API_KEY') type=str)
     parser.add_argument(
         '-l', '--label', help='enable next action labelling. Define which label to use.', type=str)
     parser.add_argument(