Update split method to swift-3-api-guidelines

Author: aciidgh

Sources/Utility/Path.swift

@@ -224,7 +224,7 @@ extension String {
     public var fileExt: String? {
         guard isFile else { return nil }
         guard characters.contains(".") else { return nil }
-        let parts = characters.split(".")
+        let parts = characters.split(separator: ".")
         if let last = parts.last where parts.count > 1 { return String(last) }
         return nil
     }