Markdown.pl: add missing tag info

Add missing info for tfoot and thead.

Add "summary" to attributes for table.

Add "col" to list of empty element tags.

Add some explanatory comments to hash tables.

Add more text to `--sanitize` help.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -2332,11 +2332,11 @@ sub _xmlfail {
 }
 
 
-my %univatt;
-my %tagatt;
-my %tagmt;
-my %lcattval;
-my %impatt;
+my %univatt;	# universally allowed attribute names
+my %tagatt;	# per-element allowed attribute names
+my %tagmt;	# empty element tags
+my %lcattval;	# names of attribute values to lowercase
+my %impatt;	# names of "implied" attributes
 BEGIN {
     %univatt = map({$_ => 1} qw(class dir id lang style title xml:lang));
     %tagatt = (
@@ -2366,14 +2366,16 @@ BEGIN {
 	'ol' => { map({$_ => 1} qw(compact start type)) },
 	'p' => { map({$_ => 1} qw(align)) },
 	'pre' => { map({$_ => 1} qw(width)) },
-	'table' => { map({$_ => 1} qw(align border cellpadding cellspacing width)) },
+	'table' => { map({$_ => 1} qw(align border cellpadding cellspacing summary width)) },
 	'tbody' => { map({$_ => 1} qw(align valign)) },
+	'tfoot' => { map({$_ => 1} qw(align valign)) },
+	'thead' => { map({$_ => 1} qw(align valign)) },
 	'td' => { map({$_ => 1} qw(align colspan height nowrap rowspan valign width)) },
 	'th' => { map({$_ => 1} qw(align colspan height nowrap rowspan valign width)) },
 	'tr' => { map({$_ => 1} qw(align valign)) },
 	'ul' => { map({$_ => 1} qw(compact type)) }
     );
-    %tagmt = map({$_ => 1} qw(area basefont br hr img));
+    %tagmt = map({$_ => 1} qw(area basefont br col hr img));
     %impatt = map({$_ => 1} qw(checked compact ismap nohref noshade nowrap));
     %lcattval = map({$_ => 1} qw(
 	align border cellpadding cellspacing checked clear color colspan
@@ -3060,6 +3062,11 @@ limited set of tag attributes will be permitted, other attributes will be
 silently discarded.  The set of allowed attributes varies by tag.
 This is enabled by default.
 
+Also split empty minimized elements that are not one of the HTML allowed
+empty elements (C<area> C<basefont> C<br> C<col> C<hr> C<img>) into separate
+begin and end tags.  For example, C<< <p/> >> or C<< <p /> >> will be split
+into C<< <p></p> >>.
+
 
 =item B<--no-sanitize>