Add Dual-Pivot Quicksort

[akerbos]

There's been a particular flavor of Quicksort in the Java library for a few years. The design space of Quicksorts is huge, sure, but I think a (basic version of) this one deserves being included in your selection.

I've implemented it in your framework here. No attribution required.

[dmcmanam]

Or perhaps add 3-pivot quicksort?