Label tooltip

[Busteren]

Exposed onMouseOver, onMouseOut and onClick events to the config. This allows adding tooltip for label quite easily.

Might include an example of this once I got time or maybe modifying the existing Demo to showcase this?

Edit*
Closes #272

[Busteren]

@jpetitcolas just thought to let you know about the PR with some additional features. I have done this on a few EventDrop charts and then when someone opened an issue I thought it would be handy to include it.

Also maybe update the NPM version. I guess the dependencies should maybe be bumped as well?

[fzaninotto]

There is already an onMouseOver property on the drop set (see https://github.com/marmelab/EventDrops/blob/master/demo/demo.js#L37 for an example). What can your PR offer that the existing code doesn't allow to do?

[Busteren]

Reverted the changelog additions. The change from * to - was done by husky and not me.
This is for exposing mouse events on the label tooltip and not the drops themselves.
This is quite handy if one has very long label names, as they can be shortened and for example, show a tooltip on hover.

[fzaninotto]

You want to show a tooltip when a user hovers on a tooltip? I'm not sure I understand. Could you be more specific about a possible use case?

[Busteren]

Meant label and not label tooltip, this PR is in response to #272 so if you have a look at that you will see what the intuition behind it is.

It exposes the onMouseOver, onMouseOut and onClick events on the line-labels. Which is very handy if you have long line-label descriptions and say want to add a tooltip.