Fix dasherize adding leading dash

Fixes elm-community#23

Author: cllns

src/String/Extra.elm

@@ -354,8 +354,8 @@ dasherize : String -> String
 dasherize string =
     string
         |> String.trim
-        |> Regex.replace (regexFromString "([A-Z])") (.match >> String.append "-")
-        |> Regex.replace (regexFromString "[_-\\s]+") (always "-")
+        |> underscored
+        |> String.replace "_" "-"
         |> String.toLower