calcurse-caldav: Add hook support

Introduce pre-sync and post-sync hooks which need to be located under
~/.calcurse/caldav/hooks/ and are executed before/after synchronization
with a CalDAV server.

Also, add an example post-sync hook and change the example post-save
hook such that it does not create tiny commits during synchronization.

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

contrib/hooks/post-save

@@ -4,6 +4,10 @@
 # automatically commit any changes to the calcurse data files using Git.
 
 cd "$HOME"/.calcurse/
+
+# Do not make commits when synchronizing with a CalDAV server.
+[ -f caldav/lock ] && exit
+
 git add *
 if ! git diff-index --quiet --cached HEAD; then
 	git commit -m "Automatic commit by the post-save hook"