simple-markdown: test `~~~~~`

Test Plan:
`make test`

Auditors: emily
simple-markdown: add `<` to escape list

Summary:
awkwardly, marked doesn't have `<` as an escapable character.
this adds it (it's nice to escape autolinks), and improves the
character escaping test, which caught this omission.

Test Plan:
`make test`

Auditors: emily