add '$' to list of escapable characters

[jjallaire]

Our markdown environment supports the inclusion of latex equations using $...$ syntax. This creates the need for users to be able to escape $ characters. Certainly understand if you don't want to take this if it changes the semantics of existing documents (e.g. $ will now render as '$' rather than '$'). If you don't want to take this as-is we could also make it a markdown extension. Of course also happy to just maintain this change on our branch.