Feature: Projectile Item Spawn Custom Subaction

https://smashboards.com/threads/custom-subaction-event-projectile-item-spawn.427019/

This could be rather easily implemented now using the ground work that was laid out from the implementation of FSM Custom subactions. fsm.py needs to be modified and a more broad "custom_subaction.py" script needs to be created to account for multiple kinds of custom events, since currently the randomizer assumes only one type of custom subaction will be used (fsm). This also involves adding support for this in "flags.py", which would probably also work similarly to how FSMs are handled.