FIX (WIP?): exclude bads from evoked plot

I noticed that in some examples the bad channels are included when `plot_evoked` is used. This fixes this. However, I think it would be better if `plot_evoked` by default does not show the bad channels, i.e., we could have a `exclude_bads` parameter which defaults to `True`. This would mean we wouldn't always have to use `picks = ..` before `plot_evoked` in order to show a clean evoked response without bad channels. Is anyone opposed to this?