Structured Formatters for JSON Output

[craven at gmx.net]

Currently there is no easy way to add support for different structured
formatters (like JSON). For example, adding support for S-Expressions
would result in code duplication.

This patch series amends the situation by introducing structured
formatters, which allow different implementations of structured output
for elements like lists, maps, strings and numbers.

The new code in sprinter.h and sprinter.c can be used
instead of the current ad-hoc output in all parts of notmuch, a patch
for notmuch-search.c is included.

In a later patch, all other parts of notmuch should be adapted to the
structured formatters, and the creation of formatters should be
centralised (to make adding new formatters easier).