Add second parameter to nl2br

Adds the optional second `is_html` boolean parameter to match the Zend
implementation. The nl2br function still needs some work to have full
parity with zend, so the zend testcases aren't moved from bad to good
yet.

fixes #1351

Edit: the current implementation only replaces `\n` using a naive implementation. In Zend `\r\n`, `\n\r` and `\r` are also replaced.