Public set placeholder API

[mnpenner]

It doesn't appear to be possible to change the placeholder on an already instantiated select2 without going through the internal API.

How we have to do it now:

var select2 = $select.data("select2");
select2.opts.placeholder = "New Placeholder";
select2.setPlaceholder();

How I think it should be done:

$select.select2('placeholder', "New Placeholder");

[kevin-brown]

Duplicate of https://github.com/ivaynberg/select2/issues/2830.