Support for `user_email` as a third type of argument that can be passed to `<user>`
Update functional test with email case
Perform `is_numeric()` checks before `is_email()`, as the former will be faster.
Support for using a user's email address in the `--user` flag.
Apparently this helper function exists
Use the fetcher instead
Merge pull request #891 from wp-cli/email-for-user-891

Support for using email address in our magical `<user>` argument