Accept custom interpolation functions

[sanniassin]

This PR extends interpolation property in VictoryLine and VictoryArea by allowing to pass functions instead of interpolation names. This gives more control over curves from d3-shape (such as specifying tension for cardinal curves) and also allows to use custom curves.

I think it fixes #561, though the proposed solution was different.

[boygirl]

@sanniassin Thanks for this improvement! These changes all look correct to me, but would you mind running nps format and commiting the changes?

[boygirl]

Thanks! I'll release a patch for you :)

[joket1999]

How can I use it ?
Like this?

<VictoryArea interpolation={d3.line().curve(d3.curveCardinal.tension(0.5))} />

Error:
ERROR TypeError: null is not an object (evaluating 'output.areaStart')

[Vemassa]

How can I use it ? Like this?

<VictoryArea interpolation={d3.line().curve(d3.curveCardinal.tension(0.5))} />

Error: ERROR TypeError: null is not an object (evaluating 'output.areaStart')

If you check the linked commit, you can see that the newly accepted interpolationFunction is actually passed as

.line()
.defined(defined)
.curve(interpolationFunction || d3Shape[${interpolationName}Closed])
...

Typically, instead of passing:
<VictoryArea interpolation={d3.line().curve(d3.curveCardinal.tension(0.5))} />
you should only do:
<VictoryArea interpolation={d3.curveCardinal.tension(0.5)} />