Add support for select/select1 option selection='open'

[tiritea]

I know ODK Collect does not currently support the selection='open' option for select/select1 controls [see https://groups.google.com/forum/#!topic/opendatakit-developers/cDWTaoFGkEM for discussion] but other XForms clients can or may. So it would be nice if there was a way for XLSForm to insert the relevant option into its resulting XML to support this. XLSForm already goes to the trouble of adding an additional option and a new question in order to support 'or_other' - this should be easy by comparison! :-)

I might suggest - in keeping with how things are done presently with 'or_other' - adding a new option to the type specifier, eg "open", which when present adds "selection='open'" to the resulting control statement. eg

     select_one colors open
     select_multiple colors open

Thoughts?

• Gareth