Improve the stop at header logic

The `summary_stop_at_header` option now causes the summary generation code
to act as follows:

 - If `summary_stop_at_header` is a number n, stop before the nth header.
 - If `summary_stop_at_header` is true, stop before the first header after
   content has been included. In other words, don't count headers at the
   top of the page.

This is a followup to ruhoh/ruhoh.rb#170.