Add support for Environment.ExpandEnvironmentVariables

There seems to be no way to use Environment.ExpandEnvironmentVariables in the parsing process to expand arguments which might need it. The only way at this stage is to do this on all the arguments passed into `Main` such as `args.Select(arg => Environment.ExpandEnvironmentVariables(arg)).ToArray()`.

It would be nice to be able to specify that an argument should be expanded.

It appears that adding middleware is too late in the process, as the Tokens are in a read-only list.