Descriptives: add extreme values table

[raviselker]

Table shows the extremest values (highest and lowest) of a numeric variable. The number of required extreme values can be defined by the user.

Important: this implementation ignores the grouping variable

Closes #168

The implementation looks as follows:

[jonathon-love]

hmm, i'm wondering if there's a more natural way of saying Extremest ... that's also compact. ... Most extreme? Min/max?