Support parsing second shifts

Parse inputs like -10s into 10 second shifts back in time.

website/sandbox/shift-time/Spec.hs

@@ -0,0 +1,17 @@
+--------------------------------------------------------------------------------
+module Spec where
+--------------------------------------------------------------------------------
+import RIO
+import Test.Hspec
+import Test.QuickCheck
+import Main hiding (main)
+
+import qualified RIO.Text as Text
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = hspec $ do
+  describe "Main" $ do
+    it "handles seconds" $ do
+      property $ \x -> parseTime (Text.concat [x & show & Text.pack, "s"]) ==
+        (Just defaultShiftTimeRequest { shiftSeconds = x })