Added random command to dfhack

This allows us to run events only some of the time. For example,
for a 25% chance of a megabeast attack:

    random 25 force megabast

This code requires DFHack r4 to run.
Enable DFHack search plug-in.

I have no idea why r4 desn't have this enabled by default.
Merge branch 'dfhack_r4' into unified

* dfhack_r4:
  Enable DFHack search plug-in.
  Added random command to dfhack