Add save_failures option

[petdance]

If you have a lot of lint failures to fix, it can be a pain to track down the actual failures. I have written far too much code that looks like this:

$mech->submit_form_ok( .. blah blah blah ... );

that I have then had to do this to:

$mech->submit_form_ok( .. blah blah blah ... ) or do { $mech->save_content( 'foo.html' ); die; }

And then you go and look at foo.html, see what the problem is, fix and repeat with the next problem.

It would be nice if we had something like this:

my $mech = WWW::Mechanize->new( save_failures => '/tmp/failure-%d.html' );

and so that would give you sequentially named files failure-1.html, failure-2.html, etc.

It would also be good if the specifics of the failure could be saved. Perhaps a CDATA section at the end of the file so as not to screw up the line numbers.