Add --all option to delete commands

[swissspidy]

Right now it's not possible to run something like wp theme delete --all or wp plugin delete --all to quickly delete all themes and plugins.

I guess my best bet right now is to use the list commands + xargs or something.

wp plugin uninstall --all works though, so I think there's some inconsistency.

Edit: #88 already covers wp plugin delete --all. My suggestion expands this to the theme command as well.

[wojsmol]

Currently I use this to delete inactive themes

wp theme install ~/test.zip --activate
wp theme delete $(wp theme list --status=inactive --field=name)

[swissspidy]

@wojsmol Simple & effective!

I wish this would work for the active theme too, see #114.

[wojsmol]

In my use-case I use custom theme so #114 is not for me but good addition in general.