Hexadecimal HTML entities are not rendered #712

According to standard Markdown, HTML entities are to be recognized and rendered correctly (https://daringfireball.net/projects/markdown/syntax#autoescape). Up to v2.6.11, Python-Markdown has been able to do so, but with v3 it does not work any longer.

A hex HTML entity is not recognized. The leading ampersand is rendered as "&", and in HTML, the entity consequently reads as the entity itself, like, for example ■ instead of the intended ■.