Add IsOptionalArg and related properties to FSharpParameter

Close #203.

Ideally, `IsOptionalArg` and the like should be exposed via attributes. Right now FCS doesn't return any attribute for `FSharpParameter` (see https://github.com/fsharp/FSharp.Compiler.Service/issues/203#issuecomment-53730889).

This PR expose `IsOptionalArg`, `IsOutArg` and `IsParamArrayArg` that are built-in attributes captured in tooltip information (e.g. https://github.com/fsharp/FSharp.Compiler.Service/blob/02303a1322ba3cb375b23ba9880174b875c3ad3a/src/fsharp/vs/service.fs#L102-L115).

It's going to unblock us in order to implement https://github.com/fsprojects/VisualFSharpPowerTools/pull/664.