Add min() and max() functions

These supplement the already existing min{} and max{} blocks, for the
case where the value to compare is itself.  Both syntaxes work for this
case now.  Previously, these would fall back to the subtle failure of
using collection's non-reactive implementation of `min` or `max`.
Merge pull request #25 from kriskowal/min-max

Add min() and max() functions