Keep high, mid, low

[soggie]

Add a syntax that allows rolling a dice pool and keeping highest, n-highest, median, n-lowest and lowest.

Syntax:

2d6kh - roll 2 d6 and keep highest score
2d6kl - roll 2 d6 and keep lowest score
2d6km - roll 2 d6 and keep medium score (in this case, will choose highest instead)
2d6kh1 - roll 2 d6 and keep highest - 1 score
2d6kl1 - roll 2 d6 and keep lowest + 1 score

Thoughts?

[rossimo]

"kh" and "kl" (with respective counts) are implemented