Consider ability to create sub-point items

[k5cents]

Right now there is no way to create lists with sub-point items. Perhaps created through a list? Right now lists aren't handled very well.

library(gluedown)
l <- list(
  a = "alpha", 
  b = "bravo", 
  c = c("charlie", "delta")
)
md_bullet(l)

• alpha
• bravo
• c(“charlie”, “delta”)

^{Created on 2019-12-11 by the reprex package (v0.3.0)}

Perhaps there should be a way to create a list like this:

• alpha
• bravo
  • charlie
  • delta

Although the l$c elements aren't really sub-points of l$b. Maybe use list names?

[k5cents]

This might not really fit in the package, which was initially intended to show R objects with markdown and not create markdown with R.

[k5cents]

Don't think there needs to be a way to create sub points, but it might be worth thinking more about potential ways to handle list objects with nested vectors.