Command to Externalize In-Line Footnotes

[polyglot-jones]

Add a command to convert all inline footnotes to externalized footnotes, by way of AsciiDoc attributes.

Some text.footnote:[Some note.]

becomes:

Some text.{fn1}

with this is added to the top of the file:

:fn1: footnote:[Some note.]

The new attribute name will be :fnNN: where NN is whatever number is the highest existing footnote number plus one.
Just in case the author has a habit of moving the exterized footnotes to a separate include file, we'll scan for all references ({fnNN}) to determine the highest number, not just definitions (:fnNN:)

Newly assigned numbers will not be zero-padded, even if the existing number(s) are.