Allow attributes to be validated against regexes

Yes, another one... I know this module supposed to be small and simple. But this is a very useful feature, and it does not add much complexity. Please reject if it is too much for you, I am happy to maintain my own fork.

I did not modify the documentation except for adding the following example to the synopsis:

```perl
# you can also specify a regex to be tested against the attribute value
my $hr = HTML::Restrict->new(
    rules => {
        iframe => [
            qw( width height allowfullscreen ),
            [ src         => qr{^http://www\.youtube\.com} ],
            [ frameborder => qr{^(0|1)$} ],
        ],
        img    => [
            qw( alt ),
            [ src => qr{^/my/images/} ],
        ],
    },
);

my $html = '<img src="http://www.example.com/image.jpg" alt="Alt Text">';
my $processed = $hr->process( $html );
# $processed now equals: <img alt="Alt Text">
```