Sign encrypted mails.

- Husk now not only encrypts outgoing emails (if possible) but also adds
  a signature.
- Rework the internal handling of the signing key.
- Integrate the signing into the workflow of the daemon.
- Add a configuration option for a subject replacement.

src/config.rs

@@ -9,13 +9,14 @@ use toml;
 use log4rs;
 
 use crate::types::errors::HuskError;
-use crate::types::defaults::CONSOLE_LOGGING;
+use crate::types::defaults::{CONSOLE_LOGGING, SUBJECT_REPLACEMENT};
 
 #[derive(Deserialize, Debug, Clone)]
 pub struct HuskConfig {
     pub connection: String,
     pub sequoia_home: String,
     pub keyservers: Option<Vec<String>>,
+    pub subject_replacement: Option<String>,
     logging: Option<toml::Value>,
 }
 
@@ -68,6 +69,13 @@ impl HuskConfig {
         }
     }
 
+    pub fn get_subject_replacement(&self) -> String {
+        if let Some(replacement) = &self.subject_replacement {
+            replacement.clone()
+        } else {
+            SUBJECT_REPLACEMENT.to_string()
+        }
+    }
 }
 
 pub type HuskConfigContainer = Arc<Mutex<HuskConfig>>;
@@ -79,3 +87,23 @@ impl From<HuskConfigContainer> for HuskConfig {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use crate::types::defaults;
+
+    use super::HuskConfig;
+
+
+    #[test]
+    fn subject_line() {
+        let config = HuskConfig {
+            connection: "".into(),
+            sequoia_home: "".into(),
+            keyservers: None,
+            subject_replacement: None,
+            logging: None
+        };
+
+        assert_eq!(config.get_subject_replacement(), defaults::SUBJECT_REPLACEMENT);
+    }
+}