Split in several tables of contributors

**Is your feature request related to a problem? Please describe.**
In the current version of the all-contributors specification and tools, all contributors are placed in the same table, regardless of the degree of involvement with the project. This makes more difficult for the users to know the number of people actively involved in the team. Also, the amount of contributions/degree of involvement is not properly recognized.

**Describe the solution you'd like**
In a project that I am maintaining, we would want to distinguish *at least* between the core team and the people who made punctual contributions. It would be even better to split the contributors in three tables: current core team, old core contributors and punctual contributors.