Emulate a right prompt in bash

[sdrik]

Fixes #2103

[PH111P]

This looks very nice!
(Although, I must admit that it does not look as "trivial" as I would have imagined it to be, so it appears I was wrong with my assessment in that regard; so all the more, thanks for your contribution!)

Some comments:

• Allowing to pass arguments from render to hl and hlstyle is probably a good idea, so although this makes this PR look slightly messy, it is a good change in my opinion. However, please add the argument (hl_args) to the corresponding documentation as well. (In particular in renderer.py.)