Add worm plot as a diagnostic device option in appraise()

[gavinsimpson]

A worm plot is effectively a rotated QQ-plot but is commonly used as a model diagnostic plot. It's widely used in the gamlss package and book for example.

van Buuren, S., Fredriks, M., 2001. Worm plot: a simple diagnostic device for modelling growth reference curves. Stat. Med. 20, 1259–1277. https://doi.org/10.1002/sim.746

Tristan Mahr also has a nice blog post on implementing these plots using ggplot2

• implement worm_plot()
• add as an option in appraise()

[gavinsimpson]

Worm plots are now implemented in worm_plot() (as of versions > 0.5.1), but not yet active as an option in appraise().