Indeterminate state for Checkbox?

[tszekely-spotcap]

Hi, this is a feature request, not an issue. I haven't seen any reference about this in the Material spec, but I've seen it implemented here:
https://material.angularjs.org/latest/demo/checkbox

It would be very nice to have this feature in react-mdl too (and I personally need it :)).

Thank you for all the good work with react-mdl, I've been using it for a while and I really didn't have any blocking bugs caused by it. :)