Update translation
Fix notices for empty checklists
Add multiple recipients and extra headers options

Something that should have been possible from the start, but I never had
the need for until now. Multiple recipients is possible with a simple
CC/BCC header, but for situations where they must be completely
separated, this new option will do multiple wp_mail calls.
Add @since docblock tags
Add documentation link to plugin meta data
Version 1.1.0
Update documentation and its generation