Handling of control characters

[dralley]

XML 1.0 disallows most kinds of control characters. XML 1.1 allows some control characters, but only when they've been encoded as numeric character references.

There should probably be some way to encode these provided at minimum, or maybe prevent the writer from writing them. I haven't gotten around to checking whether the reader checks for this yet, but libxml2 seems to throw a warning / error when it encounters raw illegal control characters.

https://en.wikipedia.org/wiki/Valid_characters_in_XML