implement function to get color frequencies

We'll adjust the spawn probability of a tile according to one minus its
frequency on the board prior to the move being effected. This will tend
to guarantee that the board is balanced, and will make the game
significantly harder.
implement color frequencies and probabilities, but not their effect

We need to adjust the probability of spawning a dot of a given color by
its current frequency in the board, to avoid the board becoming too
cluttered with a given color. The current implementation would guarantee
that a color cannot comprise more than fifty percent of the board. (At
that point, its probability becomes negative.) Actually generating the
dots is still done uniformly, but all the functions are in place. All
that remains to do is actually perform the generating.