add css properties to JSON output

[jrobinson01]

Add the css properties to the JSON output.
fixes #110

[web-padawan]

Thanks, and BTW this is something I also need in the <api-viewer> 👍

[web-padawan]

@runem would you have time to review this PR any time soon?

I was planning to implement CSS custom properties in the next api-viewer release.
This would be a really nice addition, and the proposed change seems good to me.

[runem]

Thanks for this PR @jrobinson01

I would really like to add "cssProperties" to the experimental custom-element.json format. Just to be clear: it might be removed if we land consensus on a custom-element.json format without "cssProperties" (this also applies to all other properties in the output) :-)

It might be relevant to start the discussion on adding "cssProperties" at https://github.com/webcomponents/custom-elements-json 👍

I will merge this PR and also add support for #109 when I get home from work today 🎉

[web-padawan]

@runem could you release this change so we can use it?

[runem]

I have been more caught up in work than expected this week, sorry :-/ I just released 0.1.18 with css-properties for json-format 🎉 and I will get some more time for web-component-analyzer this weekend 👍