allow supplying a format string for incrementer/decrementer and accumulator

[pstrinkle]

You might not want +{int}, -{int}, {int} as the incrementer, decrementer, and accumulator formats.

What if you wanted the accumulator to be "{int} points" or something else. You could do this via:

points to accomplish this. However, it might make sense to allow the incrementer to change, which can't presently be done. What if you wanted it to be something like "+{int} jiffies" or anything else. We can easily support this.