Initial placeholder and dynamic multi-select clickable area

Adds `initialPlaceholder` as an alternate option for the multi-select placeholder to display when nothing is selected. `placeholder` now shows as a placeholder when entries are selected, or when `initialPlaceholder` is undefined.

Use an invisible span element to determine the width of the clickable area to add new elements. This area now fits the placeholder text tightly, with a min width, and expands if the query text is long. It overflows gracefully to the next line if out of space, and allows you to click anywhere on that line (not within 40px anymore!).

Contains a couple hacks that are worth a look: `line-height: 32px` hack for the invisible text, and `width: 1000px` for the actual input field (to make sure it takes up the whole row width).

May not work very well for styled or small multi-selects; could use some thought here.

For @thangdinh 
@chrnguyen or @hanakimis may have CSS suggestions
@bigsley or @Dylnuge may have other helpful suggestions
instead of creating this invisible placeholder and the like, try giving the input in the search field width 100%. then you can style li:first-of-type.ember-select-search-field with a width of 100%, and otherwise 25px.