Add add-on support

Implement add-ons support for input groups (http://getbootstrap.com/components/#input-groups)
It is already implemented in similar (but probably dead) library here: https://github.com/magnuspwhite/zend-form-decorators-bootstrap

[Cruiser13]

This seems to be implemented:
$email = new Twitter_Bootstrap3_Form_Element_Email('email'); $email->setLabel('Email')->setAttrib('placeholder', 'Email'); $email->setAttrib('addon_append','@');