web: alternate background color per transaction, not posting, on register view

(doesn't yet work for successive identical transactions)

hledger-web/.hledger/web/templates/registerreport.hamlet

@@ -6,5 +6,5 @@
   <th.amount align=right>Amount
   <th.balance align=right>#{balancelabel}
 
- $forall i <- numbered items
+ $forall i <- numberTransactions items
   ^{itemAsHtml vd i}

hledger-web/Handlers.hs

@@ -445,3 +445,13 @@ getMessageOr mnewmsg = do
   return $ maybe oldmsg (Just . toHtml) mnewmsg
 
 numbered = zip [1..]
+
+-- Add incrementing transaction numbers to a list of register report items starting at 1.
+numberTransactions :: [RegisterReportItem] -> [(Int,RegisterReportItem)]
+numberTransactions [] = []
+numberTransactions is = number 0 is
+  where
+    number _ [] = []
+    number n (i@(Just _, _, _):is)  = (n+1,i):(number (n+1) is)
+    number n (i@(Nothing, _, _):is) = (n,i):(number n is)
+