Add support for custom toggle icons

[phidias51]

It's often useful to have two separate icons to represent the two states of the button. For example, with a iron-collapse pane it would be useful to be able to use the expand-more, expand-less icons to indicate that you can expand or collapse the pane. You might also have a lock/unlock, or WiFi on/off toggle.

[notwaldorf]

For that, wouldn't something like this make more sense? http://jsbin.com/hocumed/edit?html,output. A toggle button is more like a switch, as defined in the material design spec: https://material.google.com/components/selection-controls.html#selection-controls-switch

[phidias51]

I've done something similar in previous projects. It's a common enough use case, that I thought it would be nice to specify the icons for the two states either as properties, or via the style. Another approach might be to refactor this and extract a behavior that two implementations could use (paper-toggle button & paper-icon-toggle-button, for example).