-  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)