MarkdownMarkup: * Get rid of self.local_extensions attribute * Check if extensions exist before loading them * Add "remove-extra" and "mathjax" virtual extensions
MarkdownMarkup:
* Get rid of self.local_extensions attribute
* Check if extensions exist before loading them
* Add "remove-extra" and "mathjax" virtual extensions


mitya57/pymarkups,retext-project/pymarkups