title: Restrict input type in shiny field
body: <p>As it is, <code>numericInput</code> accepts both string and numeric inputs. If a string is entered it is converted to <code>NA</code> (try with the code below). Is there a way of not allowing the user to type a string in a numeric field in shiny?</p>
  
  <pre><code>ui &lt;- fluidPage(
    numericInput("num", label = "text not allowed", value = 1),
    verbatimTextOutput("value")
  )
  
  server &lt;- function(input, output) {
    output$value &lt;- renderPrint({ input$num })      
  }
  
  shinyApp(ui = ui, server = server)
  </code></pre>
  
  <p>So far, I have added a text output next to the numeric input that warns the user that only numbers are accepted if she enters a string in a <code>numericInput</code> field. This solution is far from ideal for me. </p>
  
  <p>I want it to be impossible for the user to enter a character value in a numeric field.</p>