Rewrite encoding functions, now it is possible to use non ascii characters in
header values and in attachment filenames. New keyword argument external-format (default :utf-8). New class attachment, with slots attachment-name and attachment-mime-type, and a constructor function make-attachment. Rename q-encode-str to rfc2045-q-encode-string. Add rfc2231-encode-string to encode non ascii characters in attachment filenames.