This introduces diffx.writer.DiffXWriter, a streaming writer for the

DiffX file format. This is a low-level interface for writing DiffX

files to a stream (such as a local file, an in-progress HTTP response,

or a memory-backed stream) progressively, section-by-section, according

to the DiffX specification.
It carefully ensures that sections are never generated in the wrong

order, using the same state tree used for parsing. It also validates

that values going into options.
The writer tends to be thorough when it generates sections, opting to

explicitly provide information like the line ending types or the

metadata format, even when optional in DiffX files.
It's up to the consumer to write the correct preambles, metadata, diff

content, and any options not otherwise handled by the writer. However,

the writer will take care to compute some things, like line ending

types, if not otherwise provided (though it's recommended that callers

still provide these if they know them).
DiffXWriter should now be usable for production DiffX generation

(pending any new changes to the spec).

Unit tests pass on Python 2 and 3.