This method allows callers to output strings with specific lengths.
It's useful both for strings with embedded NULs (which JSON can
represent, though parser support is apparently spotty), and
non-terminated strings.
Using the new structured printer support in sprinter.h, implement
sprinter_json_create, which returns a new JSON structured output
formatter. The formatter prints output similar to the existing JSON, but
with differences in whitespace (mostly newlines, --output=summary prints
the entire message summary on one line, not split across multiple lines).
Also implement a "structured" formatter for plain text that prints
prefixed strings, to be used with notmuch-search.c plain text output.
