Specify x and y crosshair scales to support non annotation based crosshair plotting

[andredumas]

Crosshair currently default's x and y scales to the first annotation element annotation arrays. This assumes there are always annotations associated. It would be better to explicitly set x and y scales, and also be able to plot crosshairs without annotations.

Before:

    var crosshair = techan.plot.crosshair()
            .xAnnotation([timeAnnotation, timeTopAnnotation])
            .yAnnotation([ohlcAnnotation, ohlcRightAnnotation]);

After:

    // Without annotations
    techan.plot.crosshair()
            .xScale(timeAnnotation.axis().scale())
            .yScale(ohlcAnnotation.axis().scale());

    // With annotations
    techan.plot.crosshair()
            .xScale(timeAnnotation.scale())
            .yScale(ohlcAnnotation.scale())
            .xAnnotation([timeAnnotation, timeTopAnnotation])
            .yAnnotation([ohlcAnnotation, ohlcRightAnnotation]);