[Idea] Support of placeholders in quick_form - similar to button_map

[ar-anvd]

Would be awesome if quick_form supports the placeholder option using a dictionary. Something exactly like button_map option.

https://pythonhosted.org/Flask-Bootstrap/forms.html?highlight=button_map#form-macro-reference

The implementation could be exactly the same as button_map, so probably is not a big deal.

Thanks