Add support to native Guid type

[delfuria]

Hi, i have tried to fork your project for make a PR but since you have used the new 2017 VS format i'm not able to do that.
So i ask you, if possible to add support to guid type simply adding a single line of code to the file

ValueArgument.cs

On method

protected virtual TValue DefaultConvert(string stringValue)

you have a lot of "if" statement for manage the native type and if you add this line of code

           if (valueType == typeof(Guid)) return (TValue)(object)Guid.Parse(stringValue);

just below the last if all works fine.
Thanks in advance

[j-maly]

Added in 3.0.13. Guid.Parse is not available in all versions of .NET, but I've added some ifdefs around that..

[delfuria]

Thanks a lot