add more std bullets (std> and std#)

[mstojanovic]

add a bit more depth choices for people that don't use alpha numerical bullets

[dkarter]

Thanks for the PR! I'm a bit hesitant to add these as I want to make sure these don't clash with markdown, asciidoc and a few other formats this plugin supports. I'd have to do a little digging and see what is the definitive standard for each of those formats + ensure no conflicts.

[mstojanovic]

this actually makes sense. so rejecting this could be understandable, but having a list of at least 2 or 3 more symbols available would be nice