Styling when disabled

[kbenjamin]

As it currently stands, there is no way to style paper-toggle-button when it is disabled.

Can you please add additional mixins to support that styling? I'm willing to do it, if you'll accept the pull request.

[bicknellr]

I think we should make the selector paper-toggle-button[disabled] work (like the CSS here) instead of doubling the number of mixins / custom properties. This would mean splitting all the mixin / custom property usages into a pair of selector:not([disabled]) and selector[disabled] rules to prevent backwards incompatibility with users that already expect the disabled state to take precedence over their values. (It looks like this might already be happening in a few places.) Feel free to submit a PR and thanks for bringing this up!