Add support for vdir synchronization

Add a script to synchronize calcurse with a VDIR collection.

Add a wrapper script around vdirsyncer to automatically synchronize
calcurse data to a vdirsyncer collection.

Add script documentation and Makefile.

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

contrib/vdir/README.md

@@ -0,0 +1,58 @@
+calcurse-vdir
+===============
+
+calcurse-vdir is a Python script designed to export and import data to and
+from directories following the
+[vdir](http://vdirsyncer.pimutils.org/en/stable/vdir.html) storage format.
+This data can then be synced with various remotes using tools like
+[vdirsyncer](https://github.com/pimutils/vdirsyncer).
+Please note that the script is alpha software! This means that:
+
+* We are eagerly looking for testers to run the script and give feedback! If
+  you find any bugs, please report them to the calcurse mailing lists or to the
+  GitHub bug tracker. If the script works fine for you, please report back as
+  well!
+
+* The script might still have bugs. MAKE BACKUPS, especially before running
+  calcurse-vdir with the `-f` flag!
+
+Usage
+-----
+
+calcurse-vdir requires an up-to-date version of calcurse and python.
+To run calcurse-vdir, call the script using
+
+```sh
+calcurse-vdir <action> <vdir>
+```
+
+where `action` is either `import` or `export` and where `vdir` is the local
+directory to interact with.
+
+When importing events, calcurse-vdir imports every event found in the vdir
+directory that is not also present in calcurse. When exporting events,
+calcurse-vdir does the opposite and writes any new event to the vdir directory.
+
+These operations are non-destructive by default, meaning that no event will be
+deleted by the script. The `-f` flag can be used to make the origin mirror the
+destination, potentially deleting events in the destination that are no longer
+present in the origin.
+
+You can optionally specify an alternative directory for local calcurse data
+using the `-D` flag if it differs from the default `~/.calcurse`.
+
+Integration with vdirsyncer
+---------------------------
+
+A vdirsyncer synchronisation script `calcurse-vdirsyncer` is can be found in
+the `contrib` directory. This script wraps event export, vdirsyncer
+synchronization and imports in a single call. Run `calcurse-vdirsyncer -h` for
+more information.
+
+Planned Updates
+---------------
+
+* Support for hook directories
+* Enable filtering of imported and exported items (events, todos)
+* Improve event parsing robustness
+* Add testing support