theme(fix): add show_recent_posts parameter to stop showing recent posts on index (#795)

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Closes #794

You can no longer set `recent_posts_number = 0` because in current hugo
versions `{{ $n_posts := $.Param "recent_posts_number" | default 3 }}`
will default to three if recent_posts_number is 0. This used to work a
couple of versions back, I am assuming this is not a regression but
rather wanted behavior in hugo -- so I introduce a new parameter
(boolean) `show_recent_posts` to enable(default) or disable recent posts
on index.



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