- Main function of this library. Parses incoming HTML, converts it into Markdown
and returns converted string. 
- Custom variant of main function. Allows to pass custom tag<->tag factory pairs
in order to register custom tag hadler for tags you want. 
- Custom variant of main function. Allows to pass custom tag<->tag factory pairs
in order to register custom tag hadler for tags you want. 
- Custom variant of main function. Allows to pass custom tag<->tag factory pairs
in order to register custom tag hadler for tags you want. 
- Same as - parse_htmlbut retains all “span” html elements intact
Markdown parsers usually strip them down when rendering but they
may be useful for later processing.