Support tooltip

[Folyd]

Fixes #141.

[mrbrianevans]

Hi there. Very nice feature idea. I would also like to use this feature.

I've tried out your PR, but the tooltips are not showing in the right place. They are offset from the mouse point and the offset changes depending on which cell I hover over. Also, I think you forgot to remove the console log on https://github.com/Folyd/svelte-heatmap/blob/30ced085f9d71cc5d583487aebd064507f10c06b/src/views/Tooltip.svelte#L12. It logs out every time I hover over a cell.

Many thanks for you contribution though!