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.

README.md

@@ -67,11 +67,19 @@ password = "password"  # Optional
 username = "reddituser1"
 title_filter = "720p|1080p"  # Optional regex pattern
 imdb_id = "tt1234567"  # Optional IMDB ID for better metadata when sending to Bitmagnet
+tags = ["movie", "hd"]  # Optional tags
 
 [sources.example2]
 username = "reddituser2"
 ```
 
+### Tags
+
+Tags are optional and can be assigned to sources. They provide additional metadata that actions can use in different ways:
+
+- **Transmission**: Tags are passed as labels.
+- **Bitmagnet**: The first tag is used as the content type (e.g., `movie`, `tv_show`), which helps Bitmagnet categorize the content properly.
+
 ## Potential future features
 
 - Support for additional BitTorrent clients (Deluge, qBittorrent, etc.)