Add pre-save and post-save hooks

This adds support for hooks which are executed before/after saving
calcurse data. Hooks can be placed under hooks/pre-save and
hooks/post-save in the data directory and need to be executable.

Potential use cases include:

* Automatically commit any changes to the data files using a VCS.
* Automatically sync with some sever component on data file changes.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/Makefile.am

@@ -20,6 +20,7 @@ calcurse_SOURCES = \
 	event.c \
 	getstring.c \
 	help.c \
+	hooks.c \
 	ical.c \
 	io.c \
 	keys.c \