Backlinking from footnotes is already the behavior of `Note` values
when writing HTML. This adds parsing support for the explicit
backlinking syntax in Textile as described here:
https://textile-lang.com/doc/footnotes
Previously, these lines would be interpreted as a new paragraph
beginning with "fn3^." in text.
* Added underlineSpan builder function. This can be easily updated if needed. The purpose is for Readers to transform underlines consistently.
* Docx Reader: Use underlineSpan and update test
* Org Reader: Use underlineSpan and add test
* Textile Reader: Use underlineSpan and add test case
* Txt2Tags Reader: Use underlineSpan and update test
* HTML Reader: Use underlineSpan and add test case
Stephen Altamirano