Ideas to improve the examples in pkgdown

[gaborcsardi]

Some of these need support from downlit as well.

• the horizontal lines can be in the manual
• separate input and output by some space
• make it easier to see what is input and what is output
• make it easier to spot the errors

(the greys should be the same greys.... and the bars should be vertical... and ignore the cursor that was caught in the screenshot)

[hadley]

Proposed class names: r-in = input, r-out = output, r-msg = message, r-err = error, r-wrn = warning.

And pr for prompt?

[gaborcsardi]

Yeah. Or r-pr?