Bring savePost under tests

[Minoru]

hakyll-convert (the executable) contains an important function savePost that writes posts to disk. We need to ensure its correctness, so we have to write tests for it. To achieve that, the function must be moved into the hakyll-convert (the library).

hakyll-convert/app/hakyll-convert.hs

Lines 115 to 117 in 202bacb

	-- | Save a post along with its comments as a mini atom feed
	savePost :: Config -> String -> DistilledPost -> IO ()
	savePost cfg ext post = do