Add structured output formatter for JSON and plain text (but don't use them yet).

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.