add cfg.plotselection to only plot those trials present in cfg.trials

I consider this a bug, but i've wrapped this in a new cfg setting to not alter default behaviour. Currently ft_spike_plot_raster.m plots a position for all trials on the y-axis, including those that have been excluded using cfg.trials. This means if you have 320 trials but only select 20 trials you still have a y-axis for 320 trials and a useless raster plot. If you set cfg.plotselection to 'yes', then the y-axis is scaled to the number of trials in cfg.trials and a normal raster plot is produced.