elementwise `min()` and `max()`

These changes implement element-wise min() and max():

mat X(10, 10, fill::randu);
mat Y(10, 10, fill::randn);

mat Z = min(X, Y);
mat W = max(X, Y);

As it turns out, the documentation already lists these functions as available even though they were not! So, no changes necessary there.

I bumped the minor version number as I plan to do a release as soon as this is merged.