Add footer

- Add a footer to each mail resulting from a split into an encrypted and
  an unencrypted version. The footer contains the recipients of the mail
  which are excluded in the corresponding stream.
- Move the construction of the `HuskContext` to the `mail`
  milter-command, as `helo` is used by scrapers to detect mailservers.
- Use `CertD::trust_root()` to get the trust root - it creates one if no
  root can be found.
- Add `chrono` dependency.

src/types/defaults.rs

@@ -8,6 +8,9 @@ pub const SUBJECT_REPLACEMENT: &str = "...";
 
 pub const PID_FILE_LOCATION: &str = "/run/husk/pid";
 
+pub const FOOTER: &str = "---\nProcessed by Husk at {date}.\n";
+pub const FOOTER_RECIPIENTS: &str = "This mail was also send to {rcpts}.\n";
+
 /// Default logging configuration.
 pub const CONSOLE_LOGGING: &str = "
 [appenders.console]