[Feature Request] Allow filter rules based on specific tag

[garritfra]

As a user, I want to be able to add a filter rule to a feed based on a specific xml tag. Currently, filtering is only possible on the title.

Concrete example

Considering a feed that has these entries:

<item>
    <title>My second post</title>
    <link>https://example.com/2</link>
    <description>Some content</description>
    <author>Foo Bar</author>
    <category>dev-blogs</category>
    <pubDate>Tue, 05 Jan 2021 01:50:00 GMT</pubDate>
</item>
<item>
    <title>My first post</title>
    <link>https://example.com/1</link>
    <description>Some more content</description>
    <author>Bar Baz</author>
    <category>news</category>
    <pubDate>Sun, 03 Jan 2021 11:00:00 GMT</pubDate>
</item>

I want to be able to only subscribe to entries marked with the category "dev-blogs".

Possible solution

Add the possibility to specify tags in Keep/Block rules. If the rule is a tag (regex/raw character parsing?), query the entire entry. If not, continue as previously implemented, by just querying the title.

Resources

This feature could be implemented here: https://github.com/miniflux/v2/blob/de7a61309878e335fe99c7afbbe6a40cc097b0ae/reader/processor/processor.go#L86

Context

I stumbled upon this issue when I wanted to subscribe to the dev-blogs of a game. The authors only have a single rss feed for all their content.