Extend `facet_grid` plot to allow setting titles with a custom template

The xarray.plot.FacetGrid.set_title method has the following signature:

    def set_titles(self, template="{coord} = {value}", maxchar=30, size=None, **kwargs):
        """
        Draw titles either above each facet or on the grid margins.
        Parameters
        ----------
        template : string
            Template for plot titles containing {coord} and {value}
        maxchar : int
            Truncate titles at maxchar
        **kwargs : keyword args
            additional arguments to matplotlib.text
        """

Typically, the facet grid plot calls these by itself, but we may want to have the option to use a different template or maxchar. Invoking this again could be made possible via the facet_grid plot function.