support UnitRegistry.setup_matplotlib #91
Description
Right now, trying to use the .plot accessor without dequantifying first will strip the units so matplotlib never sees the quantities and can't use the converter registered by UnitRegistry.setup_matplotlib(). This happens for both single plots and facet grids, either because of DataArray.values or by converting to a masked array using DataArray.to_masked_array.
cc @jthielen