VictoryBrushLine

[boygirl]

Implements VictoryBrushAxis as described here: FormidableLabs/victory#906

This PR adds the VictoryBrushAxis component which may be used in place of any axisComponent or gridComponent. In addition to the usual axis / grid line, VictoryBrushAxis will render a rectangular brushAreaComponent of brushWidth. A brushDomain may be specified which will determine an initial brushComponent to render, or a brushDomain may be selected by clicking and dragging within the brushAreaComponent. The brushDomain may also be resized by clicking within either of the rendered handleComponent areas (of width handleWidth) on either side of the brushComponent. The brushComponent, brushAreaComponent and brushHandleComponent may be customized with brushStyle, brushAreaStyle, and handleStyle respectively. Each of these components renders a Box primitive, but may be overwritten, or customized inline. When the domain of the brush changes, an onBrushDomainChange callback will fire, with arguments newDomain and props.