Added gpeu_settings_fields, gpeu_preview_link, and gpeu_email_message PHP filter hooks. Updated gpeu_email_options to pass GP Email Users as the fourth parameter.
Added security enhancement.
Added support for sending attachments. It is currently only possible to send attachments by utilizing the gpeu_email_options filter.
Added gpeu_email_options PHP filter hook to modify emails before they are sent.
Fixed issue where numeric comparisons would not work with GP Email User's conditional logic.
