add subtitles for rulebook names

[Nando Neck]

It would be nice to have the full name for rulebooks shown in the optional rules, but not in the publication section.

for this a further attribute “subtitle“ could be added

name: “Die Dampfenden Dschungel“

subtitle: “Der tiefe Süden und die Waldinseln“

displayed as: “Die Dampfenden Dschungel - Der tiefe Süden und die Waldinseln“

[Bernhard Jung]

I added a subltite property to the class RuleBook. 97d1490

The JSON structure has a property "subtitle". I already moved the subtitles there.