DFHack: Added probability-syndrome.rb - run commands with probabilities! :)

In a nutshell, this allows for probability tables to be added as
reaction products

For example the following would effectively roll 1d5,
and initiate an ambush by elves, humans, drow, dwarves,
or have no effect, based upon the result:

    [PRODUCT:100:1:BOULDER:NONE:INORGANIC:PROBABILITY_TABLE]
        [PRODUCT:20:1:BOULDER:NONE:INORGANIC:SIEGE_ELF]
        [PRODUCT:20:1:BOULDER:NONE:INORGANIC:SIEGE_HUMAN]
        [PRODUCT:20:1:BOULDER:NONE:INORGANIC:SIEGE_DROW]
        [PRODUCT:20:1:BOULDER:NONE:INORGANIC:SIEGE_DWARF]

See hack/scripts/probability-syndrome.rb for full details.

(This forms a squished commit of the side-branch `probability-syndrome`,
ending at d823b75d48ea8ce9a4e500c9b039b0c2f8313b9c)