Without this flush, if stdout is block buffered (which will happen if
it's a pipe or a file, for example) and the hook also writes to
stdout, then notmuch new's output will appear *after* the hook output.
This situation may be a little esoteric, but it's good practice to
flush before you fork anyway.
Add mechanism for running user defined hooks. Hooks are executables or
symlinks to executables stored under the new notmuch hooks directory,
<database-path>/.notmuch/hooks.
No hooks are introduced here, but adding support for a hook is now a simple
matter of calling the new notmuch_run_hook() function at an appropriate
location with the hook name.
Signed-off-by: Jani Nikula <jani@nikula.org>
