Add support for structured output formatters.

Add support for structured output formatters.

This patch adds a new struct type sprinter_t, which is used for
structured formatting, e.g. JSON or S-Expressions. The structure printer
is heavily based on code from Austin Clements
(id:87d34hsdx8.fsf@awakening.csail.mit.edu).

It includes the following functions:

    /* Start a new map/dictionary structure. This should be followed by
     * a sequence of alternating calls to map_key and one of the
     * value-printing functions until the map is ended by end.
     */
    void (*begin_map) (struct sprinter *);

    /* Start a new list/array structure.
     */
    void (*begin_list) (struct sprinter *);

    /* End the last opened list or map structure.
     */
    void (*end) (struct sprinter *);

    /* Print one string/integer/boolean/null element (possibly inside a
     * list or map, followed or preceded by separators).
     * For string, the char * must be UTF-8 encoded.
     */
    void (*string) (struct sprinter *, const char *);
    void (*integer) (struct sprinter *, int);
    void (*boolean) (struct sprinter *, notmuch_bool_t);
    void (*null) (struct sprinter *);

    /* Print the key of a map's key/value pair. The char * must be UTF-8
     * encoded.
     */
    void (*map_key) (struct sprinter *, const char *);

    /* Insert a separator (usually extra whitespace) for improved
     * readability without affecting the abstract syntax of the
     * structure being printed.
     * For JSON, this could simply be a line break.
     */
    void (*separator) (struct sprinter *);

    /* Set the current string prefix. This only affects the text
     * printer, which will print this string, followed by a colon,
     * before any string. For other printers, this does nothing.
     */
    void (*set_prefix) (struct sprinter *, const char *);

To support the plain text format properly, the following additional
function must also be implemented:

    /* Set the current string prefix. This only affects the text
     * printer, which will print this string, followed by a colon,
     * before any string. For other printers, this does nothing.
     */
    void (*set_prefix) (struct sprinter *, const char *);

The structure also contains a flag that should be set to FALSE in all
custom printers and to TRUE in the plain text formatter.

    /* True if this is the special-cased plain text printer.
     */
    notmuch_bool_t is_text_printer;

The printer can (and should) use internal state to insert delimiters
and syntax at the correct places.

Example:

format->begin_map(format);
format->map_key(format, "foo");
format->begin_list(format);
format->integer(format, 1);
format->integer(format, 2);
format->integer(format, 3);
format->end(format);
format->map_key(format, "bar");
format->begin_map(format);
format->map_key(format, "baaz");
format->string(format, "hello world");
format->end(format);
format->end(format);

would output JSON as follows:

{"foo": [1, 2, 3], "bar": { "baaz": "hello world"}}