Add support for podcasts with separate seasons

[loucasal]

Some podcasts (e.g. Alle otto della sera) have different 'seasons' and/or playlists on the RaiPlay Sound website. Currently, the feed includes the first 'season' only.

[timendum]

The script supports it, just not very clearly.

For "Alle otto della sera" you can find:

• https://timendum.github.io/raiplaysound/scrittoriallasbarra.xml
• https://timendum.github.io/raiplaysound/paradiso-gliultimi4canti.xml
• and so on

[gpz500]

It seems to me that there are podcasts which work differently. E.g. Hollywood Party - Il cinema alla radio has a "season selector" that gives access to different episodes than playlists: the season selector is organized year-by-year while the playlists are organized by argument.
In short: seasons != playlists.

It would be nice if your scripts were able to address even seasons. Just to start 😜: I noted that every season has an entry in the "filters" list in the main JSON, with filters[0] related to the last/current season, filters[1] for the penultimate, and so on.