[2.0] XML Conversion

[colinodell]

The reference parser supports rendering the AST as XML. I would like to have the same functionality in league/commonmark. This would also be a good test of using the AST for purposes other than rendering it as HTML (converting MD -> XML, HTML -> MD, etc.) However, we may need to implement the other changes in this list including the next one:

Being able to visualize the AST with an XML representation would be especially helpful for developers trying to debug custom extensions they're writing.

[colinodell]

For v2.0, I'd like to implement a simple XmlRenderer annotated with @experimental that serves as more of a debugging tool than a production output format. We can then implement full, proper MD -> XML and XML -> MD conversion in v3.0 alongside #419, since both will require some significant changes that I won't want to delay the v2 release.

[colinodell]

MD -> XML actually seems fairly straightforward, so I'm adding full, non-experimental support for that in v2.0.

XML -> MD will come later.