-  bt_form(+Contents, +Options)//
 - Emit a Bootstrap form from Contents. Each element of Contents is
one of the following terms:
input(Name, Type, InputOptions) 
select(Name, Values, SelectOptions) 
checkboxes(Name, Values, BoxOptions) 
button(Name, Type, ButtonOptions) 
button_group(Buttons, GroupOptions) 
hidden(Name, Value) 
Options processed:
- class(+Class)
 - One of 'form-inline' or 'form-horizontal'. Default is a vertical
form.
 
- label_columns(+SizeCount)
 - Number of columns of the grid to use for the label. In use for
'form-horizontal'. Default is sm-2 (a pair)