default_convert plugins metadata setting

[simonw]

{
    "plugins": {
        "datasette-media": {
            "photo": {
                "sql": "select filepath from apple_photos where uuid=:key",
                "default_convert": {
                    "heic": "jpeg"
                }
            }
        }
    }
}

default_convert can be used to default to converting certain formats. Split from #6