* Send mails as HTML as default. Setting for send as plain text.
* Add new option SendAsPlainText. remove EnableHTMLAlternative
* Send HTML mails as default
* Add html check if html2text should be performed
* Send only multipart or plain. Add deprication warning for ENABLE_HTML_ALTERNATIVE
* Still use ENABLE_HTML_ALTERNATIVE for backward compatibility
* Changed to not ignore html2text errors