Use placeholder value when no items selected

[damienmckenna]

The "placeholder" attribute seems a little useless as the "templateSelection" element is used as soon as the library loads.

It would be really useful if the placeholder could be used as the default when nothing is selected.

[damienmckenna]

FWIW I worked this out by modifying templateSelection() to check selected.length and if it's zero then load a placeholder value, otherwise use the normal string.