Add --all flag to cron event delete

[Mike-Hermans]

This is a redo of #81

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()).

This function now also includes support for the --exclude parameter added in #97.

[janw-me]

I took another look at the out of sync files and corrected them.
@Mike-Hermans feel free to take a second look.
All should be well. As far as I'm concerned this can be merged.

@schlessera can you confirm your review conflicts are now resolved?

[danielbachhuber]

Thanks for your work on this, @Mike-Hermans !

Changes have been addressed