Nivo: Add a theme option to change the color of the LineSlicesItem's line

[luisrudge]

https://github.com/plouc/nivo/blob/master/packages/line/src/LineSlicesItem.js#L36

It's fixed as #000, making it hard to use with dark backgrounds:

[acomito]

I could use this one too! I have a similar background.

[rpearce]

:100: this would be a great improvement, though passing in any number of styles would probably make it even more flexible (as is done elsewhere)

[plouc]

Fixed by https://github.com/plouc/nivo/pull/512/commits/bd7a40545229bd9f2e635ef57681df91ba90e1ce, you can customize it using theme.crosshair.line, which default to:

crosshair: {
    line: {
        stroke: '#000000',
        strokeWidth: 1,
        strokeOpacity: 0.35,
    },
}