added feature nested single expression checking

Current SimpleExprWithBraceChecker does not allow nested single expression.

For example, a expression below is not allowed by checker because braces are removable.

    def foo() = {
      List(1, 2, 3, 4).map(_ + 1).filter(_ == 1)
    }

But the nested expression is not simple and force removing braces is too restrict.

I added support for nested single expression checking and make it configurable in XML.