Configure markdown alternative styles (bullets, strong, em, hr)

[andrewbaxter]

This wouldn't be too hard to add, so if there's a large demand for it I'm happy to add it.

In general the theme of this formatter is "I don't care how it looks, I just want it to be readable and fully automatic". People who have very specific formatting preferences will at some point disagree with the formatting choices here and need to format things manually I think, so I'm not sure how useful this is.

Maybe there's a few people where just having this configurable would be enough to make it palatable for them though.