Docx reader: move combining logic to Reducible

Introduces a new function in Reducibles, concatR.  The idea is that if we
have two list of Reducibles (blocks or inlines), we can combine them and
just perform the reduction on the joining parts (the last element of the
first list, the first element of the second list). This is useful in cases
where the two lists are already reduced, and we're only worried about the
joining elements.

This actually improves the efficiency a bit further, because concatR can be
smart about empty lists.
Merge pull request #1524 from jkr/dropCap3

Docx reader: move dropcap combining logic to Reducible