Extract config file handlers into a separate file

We used custom_load_conf() to load the configuration file and
io_save_conf() to save configuration. Move these functions, including
all helpers, to a central location.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/Makefile.am

@@ -12,6 +12,7 @@ calcurse_SOURCES = \
 	apoint.c \
 	args.c \
 	calendar.c \
+	config.c \
 	custom.c \
 	day.c \
 	event.c \