Better integrate footnote marks with table cell text rendered with fmt_markdown()

[rich-iannone]

This PR ensures that table body cell text generated from fmt_markdown() can have footnote marks inline with the produced text. This fix works both for right and left placement of the footnote marks. New testthat tests were added to address these changes.

Fixes: #993
Fixes: #893

[cscheid]

I don't have enough context to give a fuller review here. If it fixes the immediate bug then that's good (hence I'm approving)

I'm just generally confused about fmt_markdown()? Is GT processing markdown on its own?

[rich-iannone]

It is processing markdown on its own with the commonmark package. This has both its advantages and disadvantages (user surprise that some things don’t render the same as in R Markdown). This is a bigger issue that we can talk about somewhere down the line.