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/vars.c

@@ -111,6 +111,7 @@ char path_keys[] = "";
 char path_cpid[] = "";
 char path_dpid[] = "";
 char path_dmon_log[] = "";
+char path_hooks[] = "";
 
 /* Variable to store global configuration. */
 struct conf conf;