Delete multiple events at once

[Mike-Hermans]

Fixes #39 by making the command more consistent with the wp cron event run command.

It is now possible to delete multiple cron events at once by either:

• Specifying multiple hooks that need to be deleted:

$ wp cron event delete wp_version_check wp_update_plugins

• Delete all cron events that are due now

$ wp cron event delete --due-now

• Delete all cron events at once

$ wp cron event delete --all

Since the selection process of the run and delete command is now the same, a separate function has been added to filter the required hooks (::get_selected_cron_events()).