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>