EHN: add 2D image for evoked plots

This is a just a small enhancement to allow user plotting the classic butterfly plots as a 2D image where the x axis is time, y is channels and color is amplitude.

In the future:
* It would be great to add an option sorting the channels from back to front so that we have a rough idea of the sensory space dynamics, but this would require a serious change of the function which currently remains unaware of the sensors' positions
* Perhaps we should add units to the colorbars
* At the moment, the image mode does not allow selecting / removing bad channels. This would probably be a requisite though, so that the default plot_evoked() and plot_evoked(plot_type='image') have similar functions.