Add method to terminate execution of story tree immediately

[Atlinx]

A solution to stopping the story would be to force all nodes to check themselves with the StoryDirector before executing. I could also instead set up a pause bool in the ProgramNode that every node checks before executing, although this would permanently terminate execution as there is no way to figure out which node was last executed. But permanently terminating execution might be a good thing -- this feature could be reserved only for throwing errors and would act as a panic button to halt all operations.