Added Footnote support using 'markdown-it-footnote'

[chmunozpardo]

As someone mentioned in #229, I added footnote support using the mentioned plugin.

I attached a screenshot with the result.

[matbun]

When will this be available for users? Thank you! :)

[chencuber]

it 's a useful feature, can you merge it and update to vscode extension, thanks!

[yodapotatofly]

Thanks for this, it helped a lot

[BillDenton]

@yzane Please release the extension with footnotes supported in export to PDF. Thanks.

[2001Y]

@yzane Is there any plan to address this issue, it would be very helpful to be able to add the markdown-it plugin manually.