allow turning the brush on/off with a redraw

[gordonwoodhull]

As demonstrated in this question:

http://stackoverflow.com/questions/41578236/disable-brush-on-range-chart-before-selecting-a-scale-from-the-dropdown-on-page

it would be helpful to be able to turn on and off the brush on demand without rendering. But currently the coordinate grid mixin only creates the brush object if .brushOn is set during the render.

Generally, laziness is the only reason for an option to work only in render and not in redraw.