`wp user import-csv` should support remote files

Use case: I'm importing users from a CSV published by Google Spreadsheet.

Right now, here's part of the output:

```
salty-wordpress ➜  wordpress-trunk.dev  wp user import-csv <url>
Warning: Missing file: $url
PHP Warning:  rewind(): stream does not support seeking in /home/vagrant/.wp-cli/vendor/wp-cli/wp-cli/php/WP_CLI/Iterators/CSV.php on line 30
```

However, the import still works. We should support remote CSV files outright.