Add pre-load and post-load hooks
Add pre-load and post-load hooks

Potential use case: Synchronize data with some server before loading.

Signed-off-by: Lukas Fleischer <c9ca73f8336dcfb037ec2c2abf52d80a6847a5cd@calcurse.org>

lfos/calcurse,lfos/calcurse,lfos/calcurse