Additions and tweaks to the khal CLI.

These are some personal aliases, shortcuts, and extensions that make (my) work with the khal / vdirsyncer command-line utilities easier and faster. Some of them may be specific to my environment and workflow, but maybe someone finds a valuable nugget in there.

Download all / some selected extensions (note that some have dependencies, though) and put them somewhere in your PATH. You can then invoke them via khal-SUBCOMMAND.

It is recommended to also use the (Bash, but should also work in Korn shell and Dash) shell functions (e.g. in your .bashrc) found at shell/wrappers.sh to transparently invoke the extensions in the same way as the built-in Kubernetes commands, via khal SUBCOMMAND.