Pango style markup formatting

Changes for 0.032 - 2023-11-01

• Pluggable elements extend parsing and rendering of elements. A pluggable element is used to implement the <img> element, to include images inline in text strings. See tests/im1.pl and tests/im3.pl for examples. Rise may now also take an 'em' or 'ex' value. 1em = 100%, 1ex = 50%.