Add option for variable fill opacity in contourf_to_geojson()

[DemetrisChr]

I added an option to specify a range of fill opacity values in contourf_to_geojson() (via the fill_opacity_range argument). It is a tuple of two numbers, min and max. The fill opacity is linearly interpolated from the range based on the contour levels. Lower contour levels have lower opacity. By default fill_opacity_range is set to None, hence ignored if it’s not specified. If fill_opacity_range is specified, fill_opacity is ignored.

This could be useful for cases where it is important to highlight the higher contours (example below).

Constant fill opacity	Variable fill opacity

[coveralls]

Coverage remained the same at 0.0% when pulling 1663eb2 on DemetrisChr:opacity-range into df4e491 on bartromgens:master.

[bartromgens]

@DemetrisChr Thanks. I just created a new release (v0.4.0) that contains this change.