Extract logic to make it usable without piston

[Farkal]

Hi and thx for the great work ! I am designing a personal assistant and i want to implement the logic using behavior tree (it would be easier for anybody to add features and to debug). For this i don't need piston but just the different behaviors, i can just fork this lib but i think we could easily extract the common logic and plug it to piston or any other lib with traits. Also making it compatible with https://www.behaviortrees.com would make the design of the behavior trees easier.