Change text of submit button

[_DorsVenabili]

Hi!, first at all, congrats for the plugin, it's a great work :)

Anyway, I was using it and I realized that I can't change the text that it shown in the submit button of the form, always is 'Submit'.

It could be a good enhancement to be able to change it, like we can change the name of the fields :)

Thanks!

[tillkruess]

Adds a filter to change the submit button text

[tillkruess]

I attached a patch which adds a filter to change the submit button text, because the » isn't that optimal in many cases.