site delete: allow deleting a site with the --url option

[35grain]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

Currently the blog ID or slug must be specified when using the command. In cases where a custom domain is used, however, the --slug option fails and it is tedious to look up the assigned site ID in an automated use case.

Describe the solution you'd like

Allow site deletion by only specifying the --url option. For example: wp site delete --url="http://example.com".

[35grain]

get_blog_id_from_url() could probably be used here as with get_id_from_blogname() and the --slug option?

[swissspidy]

We could implement wp site get <id|url> instead, then one could pair wp site delete with wp site get http://example.com --field=id. This would be more in line with all the other commands.