Support querying orders by multiple statuses

[mikejolley]

woocommerce/woocommerce#17598 (comment)

In order to change to allowing multiple statuses, the args for the endpoint would need to be changed to type => array. I didn't make this change because I didn't want to potentially break or change existing API functionality.

[ryelle]

This is already done, since we needed it for order filtering in Calypso 🙂 See #31 — is there another improvement to this you were looking for?

[mikejolley]

I didn't see it :) Thanks 👍