Add replace tagname

こんなことがしたくて、HTML::Filter::Callbacks::replace_tag を追加しましたー。


```perl
use HTML::Filter::Callbacks;
 
# tag name replaced
my $filter = HTML::Filter::Callbacks->new;
$filter->add_callbacks(
  'custom' => {
    start => sub {
      shift->replace_tag('div');
      shift->add_attr('class', 'custom-class');
    },
  },
);
say $filter->process('<custom id="my_custom" />');

# <div id="my_custom" class="cutom-class" />
```