Log Message:
lowdown: update to 0.9.1.

Version 0.9.1, 2021-10-04

Expand support for images having PHP Extra attributes to both links and
images supporting classes, images, and the existing image attributes.
These applies to both reference and inline links.

This deprecates --parse-no-img-ext in favour of the more general
--parse-no-ext-attrs. The same applies to the library's LOWDOWN_IMG_EXT
deprecated by LOWDOWN_ATTRS.