Add scripts for rss auto download

[zoriya]

Description

Allow users to define a script to decide if a rss item should be downloaded or not. If the scripts return the magic code of 80 (and regex match if it is specified), the item will be downloaded.

This PR also specifies a user agent of 'flood' for http requests used to retrieve the rss since some servers sent 403 when no user agent was specified.

Related Issue

Screenshots

Types of changes

• [ ] Breaking change (changes that break backward compatibility of public API or CLI - semver MAJOR)
• [x] New feature (non-breaking change which adds functionality - semver MINOR)
• [ ] Bug fix (non-breaking change which fixes an issue - semver PATCH)