Add support for Shadow DOM

[prantlf]

Would you accept a minimum implementation of [ShadowRoot] and [attachShadow] for the purposes of taking part in toString of HTMLElement? It helps using dom-lite in unit tests of web components. I have an implementation in 30 LOC.

It could be either added to index.js to be always available, or left outside to be required separately. The separate require would modify the prototype of HTMLElement.

const { document } = require('dom-lite')
require('dom-lite/shadow-dom') // inject the implementation

Adding it to index.js would make the usage easier, providing the tiny size of the implementation.

What do you think?

[ShadowRoot https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot [attachShadow]: https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow

[lauriro]

Show me the code! :) I updated readme with minimal contribution section.