Add support for adding ranges to axes.

[tshead2]

This would facilitate several things:

• Showing the domain without using the axis spine.
• Showing separate domains when there are multiple series.
• Showing arbitrary ranges of interest, such as the 95% HDI in Bayesian statistics.

Ranges would be displayed between the axis spine and the rest of the plot, would be stackable, and have user-controlled color / style / width / etc.