Adding `url_filter_func` configuration variable.
All URLs are now passed through the `encodeURLAttribute` function, which applies the filter as necessary. If the filter function changes the URL of an automatic link, this is reflected in the text of the link too.
Inline- and reference-style email links now benefit from the entity obfuscation since `mailto:` is now detected in `encodeURLAttribute` and it triggers the entity obfuscation.

Fixes #85.