Feat/add custom rss feed configuration

[twojtylak]

Allow custom rss feed configuration by using listByDemandAction in the PostController if settings are set (via Typoscript).

E.g. via

[getTSFE().type == 200]
    plugin.tx_blog.settings.demand {
        categories = 15,153,158,151,16,87,251,88,18,22,89,19
        categoriesConjunction = OR
        ordering = publish_date DESC
    }
[end]