[Feature] Add flags to control the direction of syncing between the servers

[JChris246]

As requested in #34, This PR adds 4 flags to control syncing between servers.

• SYNC_FROM_PLEX_TO_JELLYFIN to allow updates from plex servers to update jellyfin servers
• SYNC_FROM_JELLYFIN_TO_PLEX to allow updates from jellyfin servers to update plex servers
• SYNC_FROM_PLEX_TO_PLEX to allow updates between plex servers
• SYNC_FROM_JELLYFIN_TO_JELLYFIN to allow updates between jellyfin servers

All flags default to true, which is normal operations before this PR.

closes #34

[luigi311]

This looks perfect thank you!