Add support for polymer 3

[GermanMtzmx]

Add support for polymer 3 to avoid use : <div inner-h-t-m-l="[[ htmlContent ]]"></div>

[tomalec]

We didn't start the Polymer 3 migration for our usage yet. And given the complete shift with data binding, <dom-bind>, HTML Imports, migrating all the demos and use-cases is non-trivial.

Could you provide some code that presents your use-case - preferably in jsbin, or glitch - so it would be easier to support it.