Add audio playback with mailcap and mpv (#499)

Allow audio to also be listened to from an email. Did not include setsid because then there would be no way to pause.

share/mailcap

@@ -3,6 +3,7 @@ text/html; openfile %s ; nametemplate=%s.html
 text/html; lynx -assume_charset=%{charset} -display_charset=utf-8 -dump %s; nametemplate=%s.html; copiousoutput;
 image/*; openfile %s ;
 video/*; setsid mpv --quiet %s &; copiousoutput
+audio/*; mpv %s ;
 application/pdf; openfile %s ;
 application/pgp-encrypted; gpg -d '%s'; copiousoutput;
 application/pgp-keys; gpg --import '%s'; copiousoutput;