CucumberHtmlStream CSS and JS args should have sane defaults

[davidjgoss]

Currently CucumberHtmlStream takes 2 constructor arguments, for the paths to the CSS and JS to be inlined in the HTML. It's right that these can be customised, but they should default to the built-in CSS and JS that the package provides, rather than consumers having to know where they are and resolve the paths themselves.

In other words, I should be able to do:

const stream = new CucumberHtmlStream()

and have that include the default CSS and JS.