Dismounter fix and other

Hello,
Here is a minor update that addresses reported issues:

• OnDismount method is now called when the Dismounter interface is implemented
• Added the JSValue function that allows getting the underlying syscall/js value from a javascript value

  // JSValue returns the underlying syscall/js value of the given Javascript
  // value.
  func JSValue(v Value) js.Value {
      return jsval(v)
  }

• HTTP handler can now be set to use a light version of app.css. The light version does not provide default styling for HTML elements. It can be enabled by setting the UseMinimalDefaultStyles to true:

  h := app.Handler{
      UseMinimalDefaultStyles: true,
  }

Thanks to @jwmach1, @voldyman, and @ruanwenfeng for reporting issues and submitting PR.