Support RFC 2646 / 3676

[frederikbosch]

See https://cpbotha.net/2016/09/27/thunderbird-support-of-rfc-3676-formatflowed-is-half-broken/amp/?__twitter_impression=true and
https://tools.ietf.org/html/rfc3676

[frederikbosch]

If we implement this, it should be optional. According to my Twitter conversation this standard is not widely supported by clients and those who are not supporting will probably render the message not very well.