feat(sankey): Support complete configuration of the tooltips

[bripkens]

This is the last PR for issue #71. It will make tooltips configurable for Sankey nodes and links via two additional (optional) props:

<Sankey
        {...commonProperties}
        nodeTooltip={node => <span>Custom tooltip for node: {node.id}</span>}
        linkTooltip={node => (
            <span>
                Custom tooltip for link: {node.source.id} to {node.target.id}
            </span>
        )}
    />

In combination with the theme prop, is allows full control over tooltip content and style.

[plouc]

wow! thank you for all those contributions!
Can you just rebase this PR against the master please?

[bripkens]

@plouc: Done! :)

Any chance you could make a new release of nivo after merging this? I'd very much appreciate it :)

[plouc]

It's a little bit late in my time zone now, but I'll try to release all this good stuff this week-end.

[plouc]

@bripkens I released [email protected]

[plouc]

I've also published the storybook which now includes the new stories you added.

[bripkens]

That's awesome. Thank you @plouc!

[SouhaibBenFarhat]

@bripkens I'm a bit late guys, But I want to thank bripkens for this great MR, you saved my life man.