Better default value for pointHoverRadius option

### Feature Proposal

I'm developing a wrapper around chart.js to easily display a set of graphs from data and I realized that some things I'm implementing in it might make sense upstream in reality. Some of it is about having the "most intuitive" default values for some options so that it's as simple as possible to quickly setup charts.

This issue is a tiny proposal to improve the default value for the `pointHoverRadius` option.
From what I can see the default value is `4`, which is one more than the default value for `pointRadius`: `3`.
This makes sense and is intuitive as to make the hover radius enhance visibility of what is hovered. The thing is that if one set's pointRadius to let's say 6, then by default hovering will make the point smaller which is counter intuitive. So to keep the intuitive behaviour to have the point a bit bigger to enhance hovering one will have to explicitly set `pointHoverRadius` to 7.

### Possible Implementation

Maybe the default value for `pointHoverRadius` should not be `4` but `pointRadius+1` or `pointRadius*1.3` or something along those lines. Maybe that would be more intuitive.
This would also mean that if `pointRadius` is scripted, then `pointHoverRadius` should be a function running the `pointRadius` function and adding one to it's result. And if `pointRadius` is an array, one is added to each of it's values.

It's a bit more complex of a default value, but it could help avoiding strange issue of hovering over a point and finding it smaller and having to investigate why that is happening.

Just an idea.
In any case thanks a lot for the amazing library :)