Add alwaysShowToolTip

[CaioCoelhoChaves]

I want to use in my app a button outside the chart that shows alls tooltips in the same moment, it will be very usuful if added a param that let me do this.

[taniyarn]

@CaioCoelhoChaves
Thank you for your suggestion. Currently it displays a single tooltip, so we need to modify to display multiple tooltips at the same time. We can't get to it right away, but we will keep this Issue open.