XML Version Support

[kosidinma]

Please update to the latest version of the xml package.
Version 5.4.1 is a bit out of date.

Basically, we just need this PR to get merged => #63

[arhamcode]

You can use my fork in your project (temporary) add this in your pubspec.yaml :

webfeed:
    git:
        url: https://github.com/arhamcode/webfeed.git