Npcs that do stuff randomly

[Promises]

Some npcs such as cows, sheep and the stray dog in varrock sometimes do or say things.

this could be handled in general with an option to let npcs say something sometimes.

or with plugins that can hook into a update loop for the npcs, this could allow a lot more to be done in the future.

[TheBlackParade]

Completed by adding the NPC_INIT plugin type. :)