Support plural categories in an entry

[powerivq]

There might be multiple categories for each RSS/Atom. This PR changes the Category to an array and outputs multiple category tags.

Ref: https://validator.w3.org/feed/docs/rss2.html

category | Specify one or more categories that the channel belongs to. Follows the same rules as the <item>-level category element. More info.

[kisielk]

Shouldn't the feed itself allow multiple categories as well?

[powerivq]

@kisielk Good point. Updated to add that.

[kisielk]

@elithrar How do you think we should go about handling changes like this? Since it's a breaking API change, technically it should be a v2.0.0 under the versioning scheme? Seems weird to cut a major revision for something seemingly minor, but maybe that's just a psychological barrier.

[stale]

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