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/config.rs

@@ -21,6 +21,8 @@ pub struct SourceConfig {
     pub username: String,
     pub title_filter: Option<String>,
     pub imdb_id: Option<String>,
+    #[serde(default)]
+    pub tags: Vec<String>,
 }
 
 /// Main application configuration