load_external_url should have a proper timeout set

`requests.get` which is used by `_load_external_url` method, by default does not have any timeout set, while it's recommended for nearly all the production code to set some proper value. Currently, it's possible that `premailer` hangs forever when there are issues with a resource server.

Ref:  https://requests.readthedocs.io/en/latest/user/quickstart/#timeouts