added 'multiselected' option

Hi,

I need a way of only selecting a single point in a line chart. Currently every clicked point in a line chart keeps his "selected" class and stays highlighted.

I propose a new config option for the "selection" parameter: ```multiselected``` By default it is set to true and doesn't change the current behavior. If set to ```true``` the graph will always reset the "selected" class for all points and highlight only the last one that was clicked.

I attached a sample file to test this. Currently selection in general only seems to work for line charts (although you already have some code in place for bar charts as well). I believe this solution should work for bar charts as well, provided they ever use the ```toggleBar()``` function.

Please, let me know you what you think.