Add possibility to add a suffix to data values (typically a percent or a currency sign)

Currently, there is a "percent" option, which has two issues: 1) it is taken into account only when the datalabels plugin is enabled while it should be possible to add percents to labels and tooltips without the need to use that plugin ; 2) the sign to be added to data values should be configurable, so that percent, currency or other signs can be configured.