outline_previous variable for plugins

An {{ outline_previous }} variable for plugins that passes the outline from the Story Bible up to the section of the outline linked with the current document but not anything after that.

This will allow us to develop plugins that can do helpful things based on the story so far (without the AI knowing about things it shouldn’t know about yet).

In my experience, when given the full {{ outline }} variable, the AI will sometimes respond with things I don’t want it to know about yet because they haven’t happened yet in my story.