Add 'indeterminate' state.

[grokys]

An indeterminate state would be very useful, and in fact necessary in my case, which is that of a treeview of checkable items.

An idea for an API: an indeterminate property that when set interprets the checked property as true for checked, false for unchecked and null or undefined as indeterminate.

[notwaldorf]

Please check the discussion on this thread: #33

TL: DR; Outside of a tree-like structure, this doesn't make any sense to be added to a checkbox (most checkboxes are just a list of things). This should be a tree-checkable-element or something like that.

I've created PolymerElements/paper-elements#24 as a feature request. It hasn't been prioritized or discussed, so this is not a promise we'll implement it immediately :)