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.
Merge pull request #176 from Stebalien/improved-stop-at-header

Improve the stop at header logic