Request support for footnotes

Many Markdown processors support footnotes so it would be nice to have it in marktext.

Steps to reproduce

1. Insert into the text [^XYZ] to denote a footnote
2. Insert elsewhere the text on its own line [^XYZ]: Text for the footnote
3. When the markdown is rendered as HTML the footnotes appear at the bottom.
4. When the markdown is exported to PDF the footnotes appear at the bottom of the page.

Expected behavior: Footnotes appear at the bottom of the HTML page or corresponding PDF page.

Actual behavior: The footnote syntax is printed verbatim

Versions

• Mark Text: v 0.10.21
• OS: Mac OS 10.11.6 El Capitan