LaTeX writer: Put `~` before header in item text.

Because of the built-in line skip, LaTeX can't handle a section header
as the first element in a list item. (To be precise, it can't handle it
if the list immediately follows a section header, but the instance is
rare enough that we can afford to be a bit more general). This puts a
non-breaking space before the header to solve this problem. We won't see
this space, since the header skips a line before printing anyway.

The output is ugly in LaTeX and this structure seems like it should
probably be avoided. But it is valid HTML and native pandoc, so we
should have some sort of typesettable representation in LaTeX.
LaTeX writer: Add tests for header-in-item.
Merge pull request #1594 from jkr/itemFix

Item fix