Add footnote conversion: take 2

[cansavvy]

The sequel to #23

This is working well! I added two functions which do the work and I incorporated it to run with the bookdown_to_leanpub() function.

The footnote looks like:

Which links you to a page like this:

I think we may find we need to adjust this as it is used. I mainly made this for my use case while trying to write it guessing about other use cases but ¯_(ツ)_/¯

[cansavvy]

This error happens ocassionally and it doesn't have to do with this repository:

The downloaded binary packages are in
	C:\Users\runneradmin\AppData\Local\Temp\Rtmp4Giw5L\downloaded_packages
Error in utils::download.file(url, path, method = method, quiet = quiet,  : 
  cannot open URL 'https://api.github.com/repos/jhudsl/ari/contents/DESCRIPTION?ref=HEAD'
Calls: saveRDS ... github_DESCRIPTION -> download -> base_download -> base_download_headers
Execution halted
Error: Process completed with exit code 1.

[cansavvy]

I will re-run the tests later to see if it gets resolved (before this same error was only temporary) Nothing's been happening on https://github.com/jhudsl/ari for awhile, not sure what this is about.

[cansavvy]

I filed a separate issue but because that error doesn't have to do with these changes per se, I'm going to go ahead and merge this.