Feature Request: allow classes to be included in messagesTag etc.

[jezmck]

I was hoping to be able to set something like the following in the options to set classes on all the validation messages without having to set them on each <field-messages> tag.

{
    messagesTag: 'div.message.error',
    ...

For now, I'll use a custom HTML element, but I can't take advantage of the CSS cascade this way.

Any other suggestions are welcome