Support hyphens in tag/element css selectors

[slodge]

We're using some libraries at the moment which have hyphened tag names.

This is an increasingly common scenario because of web components and the Custom Elements spec

• see valid custom element names in https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name
• or perhaps an easier shorter to read few paragraphs intro on https://stackoverflow.com/a/22545622/373321

The current selector implementation doesn't seem to support hyphens (or some other characters like underscores) - so I'm looking at extending this - see start of work in https://github.com/rstudio/htmltools/pull/354

[slodge]

PR is now ready for review 👍