Add hooks infrastructure, basic commands (set-hook, show-hooks) and a

couple of not very useful client hooks. This will eventually let
commands be run at various points and on notifications. Joint work with
Thomas Adam.

Makefile

@@ -56,6 +56,7 @@ SRCS=	alerts.c \
 	cmd-send-keys.c \
 	cmd-set-buffer.c \
 	cmd-set-environment.c \
+	cmd-set-hook.c \
 	cmd-set-option.c \
 	cmd-show-environment.c \
 	cmd-show-messages.c \
@@ -78,6 +79,7 @@ SRCS=	alerts.c \
 	format.c \
 	grid-view.c \
 	grid.c \
+	hooks.c \
 	input-keys.c \
 	input.c \
 	job.c \