contrib/dump-asy.pl: Fix $ and & in window titles

Long description:

These would produce asy/LaTeX parse errors before.

To apply this patch, use:
curl http://cr.i3wm.org/patch/158/raw.patch | git am

@@ -32,6 +32,8 @@ sub dump_node {

     my $w = (defined($n->{window}) ? $n->{window} : "N");

     my $na = $n->{name};

     $na =~ s/#/\\#/g;

+    $na =~ s/\$/\\\$/g;

+    $na =~ s/&/\\&/g;

     $na =~ s/_/\\_/g;

     $na =~ s/~/\\textasciitilde{}/g;

     my $type = 'leaf';