underlines within links have to be escaped

@josevalim That's a first attempt to fix the issue.

In links the underline will be escaped with the ansi escape sequence code `4` (underline).
To avoid this behavior it have to be properly escaped.

cheers

PS: I couldn't add the pull-request to the already open issue, so I had to make a separat PR. https://github.com/elixir-lang/elixir/issues/1811