Skip premailer by default and only process through it if run_premailer header is present

Defines new `default_skip_premailer` configuration setting (default value is `false`) to skip premailer on all emails. This can be overrode by including the `run_premailer` header on outgoing email.

If both, `run_premailer` and `skip_premailer` headers are present, it will throw an exception on the hook.