MaxML: Matchertext-friendly XML Syntax

Goal: extend XML as conservatively as possible to provide basic support for matchertext hosting and embedding. Other than that, remain as consistent as possible with XML syntax.

Hosting:

• matchertext content tags: <name attrs [matchertext]>
• matchertext attributes: attrname=[matchertext]
• matchertext sections: <![MDATA[matchertext]]>

Embedding:

• new character entity syntax &o[]; &c{}; etc.