Add new option, footnote_link_text

`footnote_link_text` is a string that is directly inserted in
front of the footnote number. It can also be a formatted string
that the number is formatted into.

For example, if the string is "[footnote %s]", the footnote link
will be "[footnote 1]".

It will determine it the option is a forat string by running it
through `sprintf`; if the output is the same, then it's not a
format string.

Also adds tests to make sure of the new output.

lib/kramdown/converter/html.rb

@@ -314,9 +314,10 @@ def convert_footnote(el, _indent)
           @footnotes << [name, el.value, number, 0]
           @footnotes_by_name[name] = @footnotes.last
         end
+        formatted_link_text = sprintf(@options[:footnote_link_text], number)
         "<sup id=\"fnref:#{name}#{repeat}\">" \
           "<a href=\"#fn:#{name}\" class=\"footnote\" rel=\"footnote\" role=\"doc-noteref\">" \
-          "#{number}</a></sup>"
+          "#{formatted_link_text}</a></sup>"
       end
 
       def convert_raw(el, _indent)

lib/kramdown/options.rb

@@ -596,6 +596,24 @@ def self.simple_hash_validator(val, name)
       Used by: HTML
     EOF
 
+    define(:footnote_link_text, String, '%s', <<~EOF) do |val|
+      The text used for the footnote number in a footnote link
+
+      This option can be used to add additional text to the footnote
+      link. It should be a format string, and is passed the footnote
+      number as the only argument to the format string.
+      e.g. "[footnote %s]" would display as "[footnote 1]".
+
+      Default: '%s'
+      Used by: HTML
+    EOF
+      if !val.include?('%s')
+        raise Kramdown::Error, "option footnote_link_text needs to contain a '%s'"
+      end
+      val
+    end
+
+
     define(:remove_line_breaks_for_cjk, Boolean, false, <<~EOF)
       Specifies whether line breaks should be removed between CJK characters
 

test/testcases/span/04_footnote/footnote_link_text.html

@@ -0,0 +1,12 @@
+<p>This is a<sup id="fnref:ab"><a href="#fn:ab" class="footnote" rel="footnote" role="doc-noteref">[footnote 1]</a></sup> footnote<sup id="fnref:ab:1"><a href="#fn:ab" class="footnote" rel="footnote" role="doc-noteref">[footnote 1]</a></sup>. And another<sup id="fnref:bc"><a href="#fn:bc" class="footnote" rel="footnote" role="doc-noteref">[footnote 2]</a></sup>.</p>
+
+<div class="footnotes" role="doc-endnotes">
+  <ol>
+    <li id="fn:ab" role="doc-endnote">
+      <p>Some text. <a href="#fnref:ab" class="reversefootnote" role="doc-backlink">&#8617;</a> <a href="#fnref:ab:1" class="reversefootnote" role="doc-backlink">&#8617;<sup>2</sup></a></p>
+    </li>
+    <li id="fn:bc" role="doc-endnote">
+      <p>Some other text. <a href="#fnref:bc" class="reversefootnote" role="doc-backlink">&#8617;</a></p>
+    </li>
+  </ol>
+</div>

test/testcases/span/04_footnote/footnote_link_text.options

@@ -0,0 +1 @@
+:footnote_link_text: "[footnote %s]"

test/testcases/span/04_footnote/footnote_link_text.text

@@ -0,0 +1,4 @@
+This is a[^ab] footnote[^ab]. And another[^bc].
+
+[^ab]: Some text.
+[^bc]: Some other text.