These default options are used when writing a DiffX file. The options

were previously stored as hard-coded values in function definitions, but

this meant that other parts of the codebase couldn't refer to those

defaults.
This adds DEFAULT_ENCODING and DEFAULT_PREAMBLE_INDENT to

DiffXWriter.
It also adds a new options definition, MetaFormat, for available

metadata formats. This just includes JSON right now.

Unit tests passed.
Used this with the upcoming DOM change.