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/app.rs

@@ -120,6 +120,7 @@ impl App {
                             magnet_links,
                             timestamp: post.created,
                             imdb_id: source_config.imdb_id.clone(),
+                            tags: source_config.tags.clone(),
                         };
 
                         // Store the post info in the database