Add placeholder text to select and multi-select

Moved from https://github.com/Addepar/ember-widgets/pull/33. Adds placeholder customization to the ember-widgets API, making it possible to change the prompt text within the search box in both select and multi-select by adding `placeholder="prompt text"` to the select-component definition.

As far as I'm aware, this API change hasn't been discussed and needs review. If accepted, the change also needs API Documentation.

I'm in favor of the change but would suggest changing the API from "placeholder" to "placeholderText".

To review:
@bigsley @charlescharles @yayalice