Add a way to use external conditions

[Timo-Breumelhof]

Conditions for #dnncms Style Helper based a value passed from the skin:

Attribute: IfExternal, takes a string and parses it to a Boolean.

Examples:

<script runat="server">
    Function Random() As String

        Dim RandGen As New Random

        Return  RandGen.Next(0, 2).ToString

    End Function
</script>