added --environment-variables option to artisan ServeCommand

 so you can pass them through to the running server, see issue #3086

````bash
export SOME_VAR=foo
php artisan serve --environment-variables
````

For me this is useful since I've added an override to the default hostname-based environment detection routine, which is based on an environment variable (too many "environments" , I know!)

The point is just to actually specify the `variables_order` command line switch to PHP, since it misses "E" by default.

I guess you could update this to actually customise `variables_order` to ones liking, but I think `EGPCS` is fairly normal.