Support the context url option

[danmorin]

As per https://developers.fulfil.io/guides/inventory-management/#fetch-inventory-of-products-in-a-specific-location, in order to pull back a product and limit the inventory to one or multiple specific locations, you have to add a context parameter to the request. I wasn't able to find a way to do this with the ruby library.

Is there a way to do this or is it something worth adding?

[cdmwebs]

@danmorin we'll take a look. I haven't seen context used anywhere else yet.