Only label a checkbox once

By default, put the label inside the html <label>
If for some reason it's desired to go outside, specify the :label_outside option

This fixes #45

Not sure why you'd want that label outside, but it kinda looked like that was coded on purpose, and an option was suggested in https://github.com/mjbellantoni/formtastic-bootstrap/pull/39#issuecomment-10297347