Implement --sub-format in dl to set output subtitle format

The default is still SubRip SRT, but you can now change the output format to almost any of the available Codec options. There is no option to leave the subtitle format as-is yet. I.e., if there's a SRT and WebVTT subtitle, leave them both as-is.

Like always, you can configure a default in your config file, e.g.,

```yaml
dl:
  sub_format: vtt
```

Note though that SSA, SSAv4, fTTML, and fVTT are not yet supported. There are no plans to support fTTML or fVTT.

CONFIG.md

@@ -120,7 +120,7 @@ For example to set the default primary language to download to German,
 lang: de
 ```
 
-or to set `--bitrate=CVBR` for the AMZN service,
+to set `--bitrate=CVBR` for the AMZN service,
 
 ```yaml
 lang: de
@@ -128,6 +128,12 @@ AMZN:
   bitrate: CVBR
 ```
 
+or to change the output subtitle format from the default (SubRip SRT) to WebVTT,
+
+```yaml
+sub_format: vtt
+```
+
 ## downloader (str)
 
 Choose what software to use to download data throughout Devine where needed.