feat: add the concept of tags

Tags are attached to all magnet links provided by a given source, and
passed to actions. This allows for e.g. better categorization in
Bitmagnet.

src/actions/transmission/client.rs

@@ -36,10 +36,11 @@ impl TransmissionClient {
     }
 
     /// Submit a magnet link to Transmission
-    pub async fn submit_magnet(&mut self, magnet: &str) -> Result<()> {
+    pub async fn submit_magnet(&mut self, magnet: &str, tags: Vec<&str>) -> Result<()> {
         let args = TorrentAddArgs {
             filename: Some(magnet.to_string()),
             download_dir: Some(self.download_dir.clone()),
+            labels: Some(tags.iter().map(|t| t.to_string()).collect()),
             ..Default::default()
         };