Allow to set a global per_page in the configuration #503

Description of the problem, including code/CLI snippet

This is a feature request.

I would like to be able to set per_page globally in the configuration file instead of setting it for each request.

Expected Behavior

Setting per_page in http://python-gitlab.readthedocs.io/en/stable/cli.html#configuration would set per page for all request with the configured gitlab.Gitlab instance

Actual Behavior

per_page must be given everytime in ListMixin.list.

Specifications

The setting could then be defined in:

• the configuration file

• gitlab.Gitlab constructor

• ListMixin.list

• python-gitlab version: python-gitlab==1.4.0

• API version you are using (v3/v4): v4

• Gitlab server version (or gitlab.com): 9.4.5-ee