Keep highest/lowest/middle of different die sizes

[Cerxi]

Is your feature request related to a problem? Please describe.
Some games have you roll different sizes of dice and keeping a specific one of them. For example, in Sentinels Comics, one is often rolling dice of 2-3 different sizes ranging from d4 to d12, and keeping only the lowest, highest, or middle of them.

Describe the solution you'd like
Perhaps an augmentation to the dd command? If dd suported drop, keep high, keep low, and in a perfect world, a new keep middle option, and also was expanded to support higher numbers (perhaps using hex? So ddCA8 would roll a d12, a d10, and a d8?), that would work quite well.

Describe alternatives you've considered
Maybe wrapping the dice in brackets? (d12 d10 d8) kh1 for example.

(It would also be nice if km worked, keep middle, but for now dropping the lowest and then keeping the lowest of what's left works.)