Make FeedExporter customizable for export conditions

Summary

The new FeedExporter class (or rather FEEDS setting) doesn’t allow to create custom conditions on whether to export an item or not.

Motivation

Like in case with #4575, I need to customize the condition under which an item should/shouldn’t be exported. It would be nice to be able to do it like this or something in a similar fashion:

FEEDS = {
    uri: {'format': 'jsonlines', 'slot_kwargs': {'param': }}
}

class CustomFeedExporter(FeedExporter):

    def item_scraped(self, item, spider):
        for slot in self.slots:
            if item.param == slot.param:
                slot.start_exporting()
                slot.exporter.export_item(item)
                slot.itemcount += 1

Describe alternatives you’ve considered

I have to create a new FeedExporter class and override open_spider method to set custom attributes to a slot and then use it in item_scraped the way I’ve shown above.

[woho]

I think this issue can be marked as done because an item filtering feature has been implemented here in the meantime: https://github.com/scrapy/scrapy/commit/4ddc9d6b55fa708becfd70812ea44eb0c6837638 This feature will help us a lot.

[Gallaecio]

A pipeline works if you want to do this for all entries in FEEDS. But this enhancement would allow to filter differently for different FEEDS entries, which would be nice.