v4.20

• require option -c to prompt for cache encryption during oauth2 setup
• merged #138 - fix retrieval of user email for outlook-oauth2
• fixes #137 - update doc to mention outlook-oauth2
• add support for html preview command separate from html viewer command
• add support for external spell check
• attempt to detect if libetpan was built without sasl support on send failure
• add handling of gentoo dependencies in make.sh
• fixes #136 - attempt reauthentication upon oauth2 token expiration
• reinit mailimap on bad state
• update doc
• add support for forwarding email as attachment with shift-F
• use mimetype message/rfc822 when sending attached email messages
• avoid using invalid cached headers
• fixes #133 - add support for microsoft outlook oauth2
• add setup wizard support for icloud
• fixes #131 - list icloud folders correctly