Support \label and \ref in footnotes

[jgm]

Example:

The issue with cross-reference.\footnote{\label{note:X}This note will be cross referenced.}

Now I refer to a previous note in a footnote.\footnote{See n. \ref{note:X}.}

Currently pandoc just prints the label name, but we could certainly compute the note number.

[jgm]

Currently we parse the label as

Span ( "note:X" , [] , [ ( "label" , "note:X" ) ] ) []

and the reference as

Link
                ( ""
                , []
                , [ ( "reference-type" , "ref" )
                  , ( "reference" , "note:X" )
                  ]
                )
                [ Str "[note:X]" ]
                ( "#note:X" , "" )