Add watch subcommand to print play events from PMS

[glensc]

This adds watch subcommand to print play events from PMS:

➔ ./plex_trakt_sync.sh watch
Listening for events!
INFO: Starting AlertListener: ws://localhost:32400/:/websockets/notifications?X-Plex-Token=<hidden>
{'type': 'playing', 'size': 1, 'PlaySessionStateNotification': [{'sessionKey': '19', 'guid': '', 'ratingKey': '9723', 'url': '', 'key': '/library/metadata/9723', 'viewOffset': 39000, 'playQueueItemID': 17659, 'state': 'playing', 'transcodeSession': '9dd9jn2ci6cx3fynei05gs6p'}]}
{'type': 'playing', 'size': 1, 'PlaySessionStateNotification': [{'sessionKey': '19', 'guid': '', 'ratingKey': '9723', 'url': '', 'key': '/library/metadata/9723', 'viewOffset': 41000, 'playQueueItemID': 17659, 'state': 'stopped', 'transcodeSession': '9dd9jn2ci6cx3fynei05gs6p'}]}

nothing intermediately useful, other than info now.

[glensc]

Combining these events, and 'state', 'viewOffset' values, checking movie.playCount, could implement real-time synchronization.