Add support for GNU Emacs

Packages describing “emacs” as local USE flag

Package“emacs” Flag Description
app-editors/leafpadEnable Emacs key theme
media-sound/supercolliderEnable the SCEL user interface
net-mail/muInstall the mu4e mail client
net-mail/notmuchInstall Emacs UI (the main one)

All packages providing a “emacs” USE flag (85)