The new command allows pre-declaring the characters that will be seen in the yarn scripts, and provides a place to store any additional information associated with each character.
The DialogueLine.character property now returns a Character object, instead of a String.
Class UserDefinedCommand now exposes argumentString and arguments properties, allowing them to be queried in a DialogueView;
Make sure the arguments of a user-defined command are computed only once per invocation;
Added documentation for user-defined commands;
addDialogueCommand renamed into addOrphanedCommand.
