use explicit :include-atom and :include-rss flags for inclusion in feeds

[bhagany]

Following @MartyGentillon, here: #109 (comment). Intent is more clear, and this change or something like it is a prerequisite for fulfilling the goals of #109.

[martinklepsch]

Two questions:

• Can you think of situations where you'd want to render one atom-feed and another rss-feed with different items?
• This somehow couples the content task to the implementation of the RSS and Atom tasks as these know about those keys and read from them. Maybe the default-meta map should be supplied as an option and thus be extendable to the users needs? (We could still have some sensible defaults ofc)

[bhagany]

• No, I just went for maximum flexibility. I can't even think of a situation where you're using both atom-feed and rss. In any case, this implementation is just as coupled as before, it's just that in this case the coupling goes the opposite way. (in master, atom-feed and rss know about markdown, in this branch, markdown knows about atom-feed and rss)
• Yes! I have a pull request planned to modify the meta in exactly the way you describe. In the meantime, this implementation allows for overriding the default by putting it in the yaml in markdown files, or the original method of changing the :filterer on atom-feed and rss.

[martinklepsch]

Ok sounds good to me then 👍

I'll just go ahead and merge @Deraen @podviaznikov

[bhagany]

Great, thanks!

[podviaznikov]

This looks good to me. Great change.
We only need to update our Spec file https://github.com/hashobject/perun/blob/master/SPEC.md with those new properties.