Process footnotes.xml

Reference: ECMA 376 17.11

A basic initial implementation for footnotes.

* This doesn't process `endnotes.xml`
* `numFmt` is ignored and always uses decimals
* Footnotes are always placed at the very end of the document, regardless of `pos`. HTML has no concept of pages, but it could handle the beneath text option.
* `numRestart` and `numStart` are ignored, so the footnotes always start at 1, and never restarts
`as BytesIO` maybe?