Add singleton to the export list

Author: NightRa

README.md

@@ -59,6 +59,8 @@
 
     replace :: String -> String -> String -> String
 
+    singleton :: Char -> String
+
     split :: String -> String -> [String]
 
     take :: Number -> String -> String

src/Data/String.purs

@@ -11,6 +11,7 @@ module Data.String
     null,
     uncons,
     length,
+    singleton,
     localeCompare,
     replace,
     take,