Added support for multipart/alternative mails
Bumped cl-sendmail version
Merge branch 'multipart-alternative' of https://github.com/html/cl-sendmail into html-multipart-alternative

This adds proper multipart/alternative support. Thanks to Olexiy Zamkoviy.
(Closes #1.)
Bug fix: Avoid passing charset init arg to basic mail-output-stream class

Signed-off-by: Utz-Uwe Haus <lisp@uuhaus.de>
Handle unsupported mail-output-stream argument to send-email by proper condition

Signed-off-by: Utz-Uwe Haus <lisp@uuhaus.de>