Add colours to dist_spec plot

[sbfnk]

I would suggest using color instead of line types to differentiate between pmf and cmf. Since there are only two levels, the color aesthetic plays nicely with the current default of "cumulative = TRUE" for uncertain distributions.

Originally posted by @jamesmbaazam in #784 (comment)

[jamesmbaazam]

Is there a preferred color scheme here? I see ggplot2::scale_color_brewer(palette = "Dark2") is used in the main plots.