Allow duplicate selectors

[bschneidr]

Will want to allow multiple selectors sharing the exact same selector id (sel_id) and options to be placed in a given document, for the sake of convenience.

[bschneidr]

For select elements, will want to add a new attribute (something like "r_html_sel_set") that can be repeated across multiple select objects. Then the 'sel_id' argument to R and JavaScript functions will refer to this attribute rather than the actual 'id' attributes of the select object.