Markdown: enhance image URL support

When determining whether or not to add the "--imageroot" or
"--htmlroot" prefix to a relative link, ignore any query string
that may be present.  The fragment (if present) was already
being ignored.

Allow URLs given in reference lines to be wrapped like so:

   [1]: data:image/gif;base64,R0lGODlhFwAXAPMAMf///+7u7t3d3czMzLu7u6qqqp\
        mZmYiIiHd3d2ZmZlVVVURERDMzMyIiIhEREQAAACwAAAAAFwAXAAAExxDISau9Mg\
        She8DURhhHWRLDB26FkSjKqxxFqlbBWOwF4fOGgsCycRkInI+ocEAQNBNWq0caCJ\
        i9aSqqGwwIL4MAsRATeMMMEykYHBLIt7DNHETrAPrBihVwDAh2ansBXygaAj5sa1\
        x7iTUAKomEBU53B0hGVoVMTleEg0hkCD0DJAhwAlVcQT6nLwgHR1liUQNaqgkMDT\
        NWXWkSbS6lZ0eKTUIWuTSbGzlNlkS3LSYksjtPK6YJCzEwNMAgbT9nKBwg6Onq6B\
        EAOw== "title (100x100)"

This facilitates embedding small amounts of data directly in
the source without causing too much difficulty.

Update the documentation to describe this new feature.

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

Markdown.pl

@@ -539,7 +539,7 @@ sub _StripLinkDefinitions {
 			  [ ]*
 			  \n?		    # maybe *one* newline
 			  [ ]*
-			<?(\S+?)>?	    # url = $2
+			<?((?:\S(?:\\\n\s*[^\s"(])?)+?)>? # url = $2
 			  [ ]*
 			  \n?		    # maybe one newline
 			  [ ]*
@@ -556,6 +556,7 @@ sub _StripLinkDefinitions {
 	my $id = _strip(lc $1); # Link IDs are case-insensitive
 	my $url = $2;
 	my $title = _strip($3);
+	$url =~ s/\\\n\s*//gs;
 	if ($id ne "") {
 		$g_urls{$id} = _EncodeAmpsAndAngles($url);
 		if (defined($title) && $title ne "") {
@@ -2258,7 +2259,7 @@ sub _PrefixURL {
 	    $url =~ m,^//, || $url =~ /^[A-Za-z][A-Za-z0-9+.-]*:/;
     my $ans = $opt{url_prefix};
     $ans = $opt{img_prefix}
-	if $opt{img_prefix} ne '' && $url =~ m"^[^#]*\.(?:png|gif|jpe?g|svgz?)(?:#|$)"i;
+	if $opt{img_prefix} ne '' && $url =~ m"^[^#?]*\.(?:png|gif|jpe?g|svgz?)(?:[#?]|$)"i;
     return $url unless $ans ne '';
     $ans .= '/' if substr($ans, -1, 1) ne '/';
     $ans .= substr($url, 0, 1) eq '/' ? substr($url, 1) : $url;

syntax.md

@@ -1096,6 +1096,37 @@ The first dimension sets the "width" attribute and the second
 dimension sets the "height" attribute.  The dimensions are then
 removed from the "title" attribute.
 
+It's possible to wrap the url when it's specified in a reference.
+Both of these examples:
+
+      [id]: url/\
+            t\
+            o/image
+            "Optional title"
+
+      [id]: url/to/image "Optional title"
+
+Produce identical "img" tags.  Only the url can be wrapped and
+only when it's in a reference.  The backslash ("\") must be the
+last character on the line and the next line (after optional
+ignored leading whitespace) must contain at least one additional
+character that's part of the URL.
+
+This can be useful for data: urls like so:
+
+      ![image][1]
+
+      [1]: data:image/gif;base64,R0lGODlhFwAXAPMAMf///+7u7t3d3czMzLu7u6qqqp\
+           mZmYiIiHd3d2ZmZlVVVURERDMzMyIiIhEREQAAACwAAAAAFwAXAAAExxDISau9Mg\
+           She8DURhhHWRLDB26FkSjKqxxFqlbBWOwF4fOGgsCycRkInI+ocEAQNBNWq0caCJ\
+           i9aSqqGwwIL4MAsRATeMMMEykYHBLIt7DNHETrAPrBihVwDAh2ansBXygaAj5sa1\
+           x7iTUAKomEBU53B0hGVoVMTleEg0hkCD0DJAhwAlVcQT6nLwgHR1liUQNaqgkMDT\
+           NWXWkSbS6lZ0eKTUIWuTSbGzlNlkS3LSYksjtPK6YJCzEwNMAgbT9nKBwg6Onq6B\
+           EAOw== "title (100x100)"
+
+Thus allowing small amounts of image data to be embedded directly in the
+source "text" file with minimal fuss.
+
 
 - - - - -