XML Rendering

[colinodell]

This PR adds the ability to render an AST to XML. The output closely mirrors the DTD defined alongside the CommonMark spec; as a result, the output provided by this library will almost perfectly match other parsers using the same format. (We deviate from this by allowing non-standard nodes, like those found in extensions, to be included with their own attributes.)

This feature will be particularly useful in developing and debugging custom extensions.

Given Markdown input like this:

# XML Test

**[_CommonMark_](https://spec.commonmark.org) is **awesome**!**

I use the `CommonMarkConverter` everywhere I can!

---

1. item one
2. item two
    - sublist
    - sublist

The new XML renderer will produce output like this:

<?xml version="1.0" encoding="UTF-8"?>
<document xmlns="http://commonmark.org/xml/1.0">
    <heading level="1">
        <text>XML Test</text>
    </heading>
    <paragraph>
        <strong>
            <link destination="https://spec.commonmark.org" title="">
                <emph>
                    <text>CommonMark</text>
                </emph>
            </link>
            <text> is </text>
            <strong>
                <text>awesome</text>
            </strong>
            <text>!</text>
        </strong>
    </paragraph>
    <paragraph>
        <text>I use the </text>
        <code>CommonMarkConverter</code>
        <text> everywhere I can!</text>
    </paragraph>
    <thematic_break />
    <list type="ordered" start="1" tight="true" delimiter="period">
        <item>
            <paragraph>
                <text>item one</text>
            </paragraph>
        </item>
        <item>
            <paragraph>
                <text>item two</text>
            </paragraph>
            <list type="bullet" tight="true">
                <item>
                    <paragraph>
                        <text>sublist</text>
                    </paragraph>
                </item>
                <item>
                    <paragraph>
                        <text>sublist</text>
                    </paragraph>
                </item>
            </list>
        </item>
    </list>
</document>

Closes #431