Allow custom predators

Added the ability to have custom predators lines:
a eats b,Name
... where a is the predator (1 char),
          b is the prey (1 char),
          Name is the name of the prey printed if the prey is eaten

Changed predator macro to #predator record
Added #move record

Renamed functions for clarity:
  position -> river_desc
  prey -> unsafe_prey
  position_and_moves -> parse