Limit rules to templates tagged with lit-html's html tag

I'm introducing Lit Element into [my Polymer 3 project](https://github.com/home-assistant/home-assistant-polymer/pull/1738). I won't be able to convert all elements to Lit from the get go and so have a mix of Polymer 3 and Lit Elements.

The issue is that the rules will scan based on finding a template literal expression started with a `html` tag, however that is also how Polymer 3 functions ([example](https://github.com/home-assistant/home-assistant-polymer/blob/master/src/panels/lovelace/components/hui-image.js#L24).

A fix for this would be to add a check that the html tag that is used was imported from the lit-html package (lit-element re-exports html from lit-html).