Implement new util for string

Allow to do string.format with named placeholders like {{placeholderName}}

The API look like this : 

    Util.String.NamedFormat( "Hey {{username}}, I'm {{me}}. How are you ?", new { username = "babar", me = "shabinou" } );

And the result will be : 

    "Hey babar, I'm shabinou. How are you ?"