Add support for indeterminate state

[jouni]

To be as usable as a native checkbox input, paper-checkbox should support the indeterminate property, as in the state when the value is neither true or false, which is visually commonly indicated with a dash inside the checkbox element.

Example of a native checkbox input in the indeterminate state

The state is needed for instance in a data table with a “select all” feature, where the checkbox that toggles the selection of all/no items should indicate if there are some items selected in the table.

[notwaldorf]

We've had an in depth discussion about this on this thread as to why this shouldn't be a property of the checkbox, but a separate element in itself: #33

The feature request for a checkable node element is here: PolymerElements/paper-elements#24

[jouni]

Ah, sorry about not searching old issues first, my bad. Thanks for the response!