Merge plex_library_sync.py into plex_sync.py

[Casvt]

In what file should the feature be added?

plex_sync.py

What is the feature request?

plex_library_sync.py can be seen as syncing two libraries on the same server. So in order to make this work, a few things need to be added to the plex_sync.py script:

1. Better target selection. Instead of syncing the whole server, there needs to be options to only sync a certain library, or even specific media only. This way you can specify to only sync a certain library
2. If needed, allow the source and target server to be the same server. Add optimisations that can be made if they are the same, e.g. only fetching library data once and save it as both the source and target data.
3. Allow manual overriding of the library mapping. Currently, libraries are said to be the same on two servers if they're for the same media type and if they have the same name. Allow to override this, so that the user can specify that "Movies" and "Movies (Kids only)" are the "same" library on the same server.
4. Add any handling for if the media is not found.