Parameter class missing support for Descriptions and and Placeholders

[mbreissi]

Need to add support and methods for setDescription() and setPlaceholder()

[mbreissi]

Suggested implementstion in Parameter.cs:

	    public string Description
	    {
		    get => this["description"].Value<string>();
		    set => this["description"] = value;
	    }

	    public string Placeholder
	    {
		    get => this["placeholder"].Value<string>();
		    set => this["placeholder"] = value;
	    }

	    public Parameter SetDescription(string description)
	    {
		    Description = description;
		    return this;
	    }

	    public Parameter SetPlaceholder(string placeholder)
	    {
		    Placeholder = placeholder;
		    return this;
	    }