Add podcasting 2.0 namespace to unsupported namespaces

[Marzal]

So it doesn't show a warning like #84

Example feeds for testing: https://kdeexpress.gitlab.io/feed https://kdeexpress.gitlab.io/feed_mp3

[dontcallmedom]

how popular is that namespace? ignoring it is naturally an option, but if indeed it is popular, it might be worth supporting it properly - compared to schema.org in #84 I assume it's not a huge and moving target.

(if so, the right approach might to be merge this and file a separate issue to consider properly supporting the namespace)

[Marzal]

It would be a better approach without a doubt, in the podcasting business is getting a lot of traction here you can check some apps and hostings publicly supporting the namespace (Blubrry, RSS.com, PeerTube, RSS Blue ...). I did this MR because I'm not a developer, so this was all I could do for my self.

The namespace is right now mature, and not very large, there are some new proposals but still no plan to change the namespace in the near future.

Here are some other feeds validators that already support it:

• https://podba.se/validate/
• https://castfeedvalidator.com/
• https://validator.livewire.io/ - This one is the more advanced and open source

So if anyone is in the mood and have the time to fully implement, it would be great, I can help in a not profesional dev capacity :sweat_smile: