Allow option to replant on piston events.

[captncraig]

Most tree farm designs involve pushing a tree with pistons after it grows. It would be awesome if TreeAssist had an option to listen to piston events and replant a sapling if the bottom log is pushed by a piston. This would allow fully automatic tree farms. It would probably be best if enabled by an option, since some server admins might not want that.

[FriendlyBaron]

"How hard can it be!"
I'll try this, been messing around with pistons for another plugin already. This also reminds me I need to start pushing to github again.