Make selectorClass accept function

[niksy]

If selectorClass accepts function as value, it could be used for more expressive checks, e.g.

<drag-select-container :selectorClass="vm => vm.$el.hasClass('item')">
	<template scope="{ selectedItems }">
		<!-- ... -->
	</template>
</drag-select-container>

But with this name selectorClass is not descriptive enough. Maybe rename it to something like selectableItem?

[stephan281094]

Great idea! This should definitely be implemented. I'm not sure about the new name though. I agree that selectorClass needs to go, but is selectableItem clear enough? (cc @FrancisKong)

[FrancisKong]

What about selectableItems is an array of nodes or components?