Banzai::Filter::AutolinkFilter is slow

This filter is used for automatically turning certain strings into URLs. As per http://performance.gitlab.net/dashboard/db/markdown-filter-methods?var-process_type=rails&var-method=Banzai%3A%3AFilter%3A%3AAutolinkFilter%23call this filter can take between 100 ms and 580 ms, with some peaks even higher than that. For part of the work we use a library called Rinku, as a start we should instrument this to see how much time is being spent there.