SetInitialVariables improvements

[alexjhetherington]

Method SetInitialVariables in class DialogueRunner

1. This method is currently uncalled; I propose that it should be called when a story is started inside DialogueRunner, which solves the issue of autostarted stories missing variables
2. It could be refactored to use method SetAllVariables on VariableStorageBehaviour, which is helpful if a custom variable storage object fires events when variables are set

[dogboydog]

Hey, thanks for the issue. Looking at the Unity implementation which this repo is based on, I don't think this is used there either. Seems to be a utility provided for users to manually reset values to initial variables as far as I can tell.

I haven't seen the issue with auto started stories missing variables, could you make an issue describing that? Seems that the root of the request here is fixing that, which might just be a bug with another cause.

[dogboydog]

I just added an example usage of 'start automatically' in #16, where Intro.yarn seems to use the default value of the string variable correctly. But if you can give a reproduction of the scenario you're seeing where variables aren't initialized, I can take a look. Thank you