Add ability to display tooltips on multi-lines

When data labels are too long to get displayed entirely on a chart, it's handy to get them displayed entirely at least in the tooltips. In order to do so, it might me necessary to display the tooltips on several lines, depending on the width available. Hence I suggest adding the a parameter "maxCharactersPerLine" to the "tooltip" options: when set, the tooltip title will be splitted in several lines with at most the given number of characters per line.