rapidjson

PrettyWriter formatting options.

This change adds PrettyWriter::SetFormatOptions with a corresponding
bitfield enum PrettyFormatOptions. This allows options affecting the
format of the PrettyWriter to be set. The first option to be provided
is kFormatSingleLineArray, which instructs the PrettyWriter to write
arrays on a single line, rather than breaking them up onto a line
per element.