implements "The Trident of Fish Command"

![screenshot](http://i.imgur.com/f3DrBqF.png)

* The trident of fish command is a terrible artifact.
* The trident is fishbane, and grants you fish-specific
  detection while you wield it.
  (fishiness is determined by the M1_SWIM flag)
* Technically, it grants the extrinsic effect Detect_fish via
  a new artifact intrinsic called SPFX_FISH
* The trident mostly sucks and just increments the artifact
  counter as a subtle screw you to the player, though
  it might be handy on one particular dungeon level.