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 structures like lists, maps, strings and numbers. The new code in structured-output.h and structured-output.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).