Entity encode mailto anchors

Previously the following was left unencoded: [send feedback](mailto:feedback@example.org).

This commit introduces a public Markdown::entityEncodeText (static) function since we need to call this from an anonymous function and probably want to keep compatibility with older PHP versions (which lack closures). Though if the public function is undesired, we can drop the anonymous function and pass array($this, «function name») as callback, which seems to be the current convention.