Add support to footnote

[rgaiacs]

Some Writing Style Guides use footnotes to reference pieces of work in the text, e.g. Chicago Style Guide. For authors that follow this style, support to footnote is important. Pandoc’s Markdown allows footnotes, using the following syntax:

Here is a footnote reference,[^1] and another.[^longnote]

[^1]: Here is the footnote.

At the moment footnote added in Word are strip when converting to Jupyter Notebook.