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

[dogboydog]

Since I haven't had any reports outside this issue of missing variables, and this method was not called directly in the Unity version either, I'm going to close this. Let me know if you have any other issues