[onion] Configurable time frames

[alpgarcia]

Currently onion study is computed by quarters. A nice enhancement would be supporting configurable time intervals.

Possible use cases:

• Use another time unit supported by Kibiter/Kibana (Month, Week, etc.)
• Use more flexible dates, for instance milestones.

Implementing one or another (or both) is something to be discussed as the latter one could present a real challenge in terms of configuration.

Ideas and proposals are welcome. For more information on the use case behind this, please see the original GitLab team request at: https://gitlab.com/Bitergia/c/gitlab/support/issues/54