How to dynamically create selectinput with multiple selection option

I have two selectinput fields and a button as below,

Column(3,selectInput("attribute","Filter by:", c("All","Clear","Exchange"),multiple=TRUE))
Column(3,selectInput("attributeval","Filter value:", c("All")))
Column(2,actionButton("addmultifilter","+"))

While clicking the '+' button, I want to dynamically create both selectInput fields right below the current two selectinput fields. Also how to capture those multiple values from selectInput.

I am new to R. Can someone give any idea for implementing this.