Add XML support

[doug-jacob]

Closes #34

The MDN Web API for DOM parsing is now supported with the addition of xmldom:
https://github.com/xmldom/xmldom

Users can now parse and edit XML documents:

const dp = new DOMParser();
const xmlDoc = dp.parseFromString("<p>some text</p>", "text/xml");
const firstElement = xmlDoc.firstChild;
const newElement = xmlDoc.createElement("span");
newElement.textContent = "some new text";
firstElement.appendChild(newElement);
const xs = new XMLSerializer();
const output = xs.serializeToString(xmlDoc);

// expected output: <p>some text<span>some new text</span></p>
console.log(output);

[oconnor0]

Looks good. Just needs a bit of documentation.

Not exactly sure I can meaningfully review Vender/xmldom, but then I don't fully review other third-party deps.

[oconnor0]

This may need to become a more formal thing at some point to support IPlugin but is great for now.