Update html2pango to 0.5

This adds support for more types of markup:

• Superscript (<sup>) and subscript <sub>
• Horizontal rules (<hr>)
• Text foreground (data-mx-color) and background (data-mx-bg-color) color

It also fixes space collapsing in block elements such that newlines in HTML are no longer rendered as line breaks and fixes support for <br>.

Closes #871 (closed), #872 (closed).