References in text are now converted into hyperlinks

References are bracketed---they are converted into superscripts. If text
immediately preceeding the reference is bracketed, it is also hyperlinked.

msgfmt

@@ -34,8 +34,14 @@ awk '/^diff --git/ { exit } { print }' \
     # happen
     s#[fh]ttps\?://[^]\n )]\+#<a href="&">&</a>#g;
 
+    # reference definitions (footnotes)
+    s#\n\[\([0-9]\+\)\]#</p><p id="ref-\1">&#g;
+
+    # references in text
+    s|\[\([^]]\+\)\]\[\([0-9]\+\)\]|<a href="#ref-\2">\1</a>\[\2\]|g
+    s|\[\([0-9]\+\)\]|<sup><a href="#ref-\1">&</a></sup>|g
+
     # paragraphs
-    s#\n\[[0-9]\+\]#</p><p>&#g;
     s#\n\n#</p>&<p>#g;
 
     # basic formatting