Feature request: have checkbox settings return value instead of boolean #159

Summary

I would like a setting with type 'checkbox' that returns the value of the setting instead of the boolean 'checked'

Step to reproduce

create a setting with input type checkbox. Have a field for a return value ('output-value'). When the setting is checked in client-view the placeholder ([[++mysetting]]) should return the value given in the output-value. instead of a boolean 1 or 0. This behaviour is similar to a MODX TV with input type checkbox

Expected behavior

a value given as output value should be returned when the checkbox is 'checked'