Implement emph parsing rule change
Implement emph parsing rule change

This implements the rule that we can't have emphasis matches
when (a) one of the delimiters can open AND can close, and (b)
the sum of the lengths of the delimiter runs containing open
and close delimiters is a multiple of 3.

Mirrors jgm/commonmark.js#fd21a0a

thephpleague/commonmark,thephpleague/commonmark,colinodell/commonmark