Add ability to set VictoryLegend symbol size via theme propery

[danny-andrews]

It looks like the size of the symbol in a VictoryLegend component is either derived from the font-size or from the symbol.size property of the datum. It would be nice if this was configurable via the VictoryTheme, as is symbol type.

Probably something as simple as:

legend: {
    style: {
      data: {
        type: 'square',
        size: 16,
      },
    },
  }

And then in victory-legend/helper-methods.js:

size: dataStyle.size || getSymbolSize(datum, fontSize),

I'd be happy to make a PR if this seems like a good approach.

[boygirl]

@danny-andrews

Good timing. I'm actually in the middle of a big legend overhaul. I'll make sure this functionality gets in.

[danny-andrews]

Splendid. :)