Allow colormap as string argument

Plot.img_plot requires supplying a color map instance. It's convenient (à la matplotlib) to instead be able to specify a default color map as a string name. `color_map_name_dict` already defines a mapping from string name to color map function in `chaco.default_colormaps`.

This seems like an easy patch if limited only to `img_plot`, but might be more work to handle for all other plot helpers on `Plot` since the parsing of keyword arguments does not appear to be handled consistently.