Allow ignore tags, attributes and generic errors

Hi,

I've added some settings to html validation when we want to skip/ignore specific errors, this might be handy
```ruby
HTMLValidation.ignored_attribute_errors = ['tabindex'] #ignore errors that attribute
HTMLValidation.ignored_tag_errors = ['inline'] #ignore errors that match tag
HTMLValidation.ignored_errors = ['missing li'] #ignore errors that match string
```