* src/gpgoladdin.cpp (check_html_preferred): New. Read Outlook's
setting.
(GpgolAddin::OnStartupComplete): Check html state
* src/mail.cpp (Mail::decrypt_verify): Use plain placeholder
if html disabled.
(Mail::update_body): Use correct parts.
* src/main.cpp (read_options): Don't read preferHTML.
(write_options): Don't write preferHTML.
* src/parsecontroller.cpp: Add plain text template.
(format_error): Use it.
(ParseController::parse): Use it.