For most `wp user` subcommands, the first argument can be user ID or login
Update documentation
add missing docs for `user add-role`; improve docs for `user set-role`
Update to accommodate scenario when multiple users are passed

See https://github.com/wp-cli/wp-cli/pull/555#discussion_r5179695
Merge branch 'master' into user-first-arg
Update "Generating Users" scenario to accommodate deleting multiple users in one go.
Merge branch 'user-first-arg' of github.com:wp-cli/wp-cli into user-first-arg
rename get_user_from_first_arg() to get_user()
don't abort if a user isn't found
Merge pull request #555 from wp-cli/user-first-arg

`wp user` subcommands should accept `user_login` or `user_id`