add support for headless chrome

[bhurlow]

now that chrome is supporting a --headless mode, figwheel support for this feature could open up some interesting possibilities including:

• running your tests in a real chrome context without opening a new window
• having REPL access to a real live DOM
• tests?

Though this may not be too much of a gain for local development (how hard is it to open a browser window?), this is something I might use in a CI environment where it's not possible to click and navigate to a browser window and another js context like Node/Nashorn does provide a full picture.

Thoughts?

[bhauman]

As far as I know nothing needs to be done to support this

[bhauman]

Going to close this until it is known that we need to do something specific to support headless chrome.