markdown characters not escaped in new editor

[tech4him1]

- Do you want to request a feature or report a bug?
bug, caused by #254

- What is the current behavior?
If a user types (#) pound character(s) at the beginning of a line in the regular editor, then switches to the markdown editor (and edits anything), the editor will convert the line to a heading.

- If the current behavior is a bug, please provide the steps to reproduce.

1. In the regular editor, add a line like this: ## this is a test.
2. Switch to the markdown editor and change any line (doesn't have to be the same line).

- What is the expected behavior?
The pound sign should be escaped, like in the old editor (\## this is a test).
We should probably check all other special MD characters as well.

[erquhart]

Great catch - we need to escape any markdown characters entered in the visual editor, we're currently escaping none of them.

Characters to always escape: _, *, (backtick), [
Characters to escape when at the beginning of a block: #, -
Characters to escape when more than one present: ~