New option for the block/mark to have noCloseToken

[rowanc1]

This allows extension into other libraries that may other markdown tokens that are not opened or closed. For example, texmath.

See ProseMirror/prosemirror-markdown#43

Notes:

• This is backwards compatible
• I have not added any tests
• I added the property to the documentation

I am not sure how to update type definitions. Let me know if you would like to see any changes on the PR!

[marijnh]

Thanks! Released as prosemirror-markdown 1.5.0