Add offset and limit to get_packages()
Add offset and limit to get_packages()

When an account surpases 20 packages, the default values that were implicitly
used in the API call (offset = 0, limit = 20) are no longer sufficient for
the client to access all packages. Exposing these supported API parameters
allows clients with more packages to access them all.

API docs on pagination: https://github.com/xplenty/xplenty-api-doc#user-content-Collection

xplenty/xplenty.py