Implement non-interactive edits

[lukasl-dev]

Previously, khal only supported interactive edits. My commits adds the feature to specify edit flags, such as --summary and --description, to edit an event non-interactively (interesting for automated environments, such as cli, scripts, agents, etc.).

If none of the new edit flags is specified, khal runs in interactive mode as expected before my changes.

If there are multiple matches, the changes error by default. To change all matches, specify --all.

Updated command description:

Edit (or delete) events matching the search string.

When run without any of the edit flags (--summary, --description, etc.),
this command runs in interactive mode, allowing you to edit events one at a
time.

When run with edit flags, this command runs in non-interactive mode. If only
one event matches the search string, it is edited directly. If multiple
events match, the command will error out unless --all is used.