FacetGrid.set_title should allow more extensive formatting

Is your feature request related to a problem? Please describe.
I've been using facetgrids fairly extensively with data that is a numpy.float32.

Using set_title does not lead to properly formatted results, as this dtype does not match isinstance(x, (float, np.float_)), so it is cast directly to a string leading to labels that are often incredibly unreadable.

Describe the solution you'd like
Do not use the format_item in the facetgrid.set_titles function for non-datetime values, but instead allow arguments of the form "{value:0.4f}" etc

Describe alternatives you've considered
The alternative is that format_item should be loser on it's isinstance checks, in particular allowing other precisions of float to be formatted. Ideally, both of these could be implemented in tandem

Locate FacetGrid.set_title/set_titles and the format_item logic, then inspect existing plotting tests for title formatting. Check how non-datetime values and numpy.float32 values are currently handled. Done means format specifications such as "{value:0.4f}" produce readable facet titles without breaking datetime formatting.