Better Catalina support thanks to @dsedivec

Catalina support (untested as I don't run Catalina myself)

Mojave support

Official 2.9.0 release.

Bugfix

Minor changes

Initial High Sierra support.

Long overdue release of v2.8.0 (no new features since latest 2.8.0 alpha release)

Support for Mail 10.3 (macOS Sierra 10.12.4)

Fixes #77, and also brought back the removal of attachment placeholders in Sierra.