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>
767214e4f1