Log Message:
lowdown: update to 0.9.0.

Version 0.9.0, 2021-09-23

Metadata consistency. Make sure that all standalone document metadata
(e.g., css for HTML, volume for manpages, etc.) is properly escaped
for its output context.

The -X option (metadata extraction) now extracts the unformatted
version of the given key. This is because output media don't
necessarily have a single way of formatting data: in -Thtml, for
instance, metadata may be formatted differently if it appears in
links, attributes, or within HTML content. It's now the caller's
responsibility to escape the results of -X.

This generates API changes, because metadata is now recorded during
the parse phase, not during the rendering phase. While here, metadata
extraction for "diff" mode is disabled (as it should be).

While here, the "base level header" metadata key has been replaced
with "shift-heading-level-by". This is because the existing "base
header level" never actually worked properly, and instead behaved
like "shift-heading-level-by".

Add GFM task lists to all output modes. While doing this, clean up
list output for -Tgemini and -Tterm.