Support for new plex credits detection feature

[silencebow]

Plex has just released a new credits detection feature outlined here and here that doesn't appear to work in plex-mpv-shim. I tested both a series and a movie that had working credits in the plex desktop app for windows but didn't work with shim.

Based on the XML it appears credits are handled similarly to intros so hopefully implementing support wouldn't be difficult.

Credits:

<Marker final="1" id="53645" type="credits" startTimeOffset="1331542" endTimeOffset="1415047">
<Attributes id="53645" version="4"/>
</Marker>

Intros:

<Marker id="53645" type="intro" startTimeOffset="17208" endTimeOffset="105939">
<Attributes id="53645"/>
</Marker>

[calmcacil]

+1 this would be a welcome addition, when using the plex client itself having credit skip is quite nice in some cases so having it with shim as well would be great.

[Brett824]

Wasn't sure if @iwalton3 was going to implement this so I threw up a PR with my implementation. Should work identically to intro skipping if that PR gets merged.