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.
Beamer:  Use \footnote<.->{..} for notes.

This ensures that the footnotes will not appear before the
overlays in which their corresponding note markers appear.

Closes #1525.
Markdown reader: Improved parsing of indented code in list items.

Indented code at the beginning of a list item must be indented eight
spaces from the margin (or from the edge of the container), or four
spaces past the list marker, whichever is farther.

Some examples in `tests/markdown-reader-more.txt`.
Merge pull request #1524 from jkr/dropCap3

Docx reader: move dropcap combining logic to Reducible
Reorganized changelog.
Updated changelog.
Minor changelog reformatting.