Adds kramdown footnote link schema to Sanitize config

Kramdown footnote anchors are generated with id's containing colons, eg:
\#fn:1 or #fnref:1

Sanitize is seeing those as links with dodgey protocols (eg. href="javascript:kill_everthing()").
This commit adds the footnote id prefixes to Sanitize's list of allowed protocols to get round the
issue.