Add Monoid import for older versions of base

src/CLI.hs

@@ -7,6 +7,7 @@ import Options.Applicative (
     Parser, execParser, flag, flag', fullDesc, header, help, helper, info, long
   )
 import Control.Applicative ((<**>), (<|>))
+import Data.Monoid ((<>))
 import Data.Version (showVersion)
 import qualified Paths_pilu as Pilu (version)