ENH: SSP toggle box for plot evoked

Here it comes, the first usable version.
Currently I'm having an issue with the EEG average electrode SSP vector, but it seems this is not related to this PR but rather an independent bug. The rest works fine, everything reversible ;-)
@Eric89GXL I made use of a few patterns you contributed for the plot_raw but I finally decided not merge everything into a super-general scheme. It turned out I that I only needed a small proportion if the plot_raw private functionality to achieve what I wanted.