add a merge_empty_sections flag to merge debateSection titles

if no child nodes are found, we merge the headings (by default,
you can pass in merge_empty_sections: false if you don't want
this behaviour.)

This is to prevent:

a) multiple titles in a minister's response.  e.g.

    VERY IMPORTANT TOPIC #1
    VERY IMPORTANT TOPIC #2
    (Minister's Response)

b) headings oddly parsed as 2

    1. REPORT OF THE PORTFOLIO COMMITTEE ON HUMAN SETTLEMENTS ON AN OVERSIGHT
    VISIT TO LENASIA, GAUTENG, DATED 22 FEBRUARY 2013

c) (possibly not ideal) prevent empty sections where we're simply not
inferring structure correctly.

    ANNOUNCEMENTES, TABLINGS, AND COMMITTEE MINUTES
    MEETING DATED X