Add a fast mode

[Ullaakut]

• Add an option --fast (maybe turned on by default?) for big repositories (>2K stars) where Astronomer would
  • Fetch the list of stargazers first without querying user data
  • Select 50 random slices of 20 users within those stargazers
  • Compute the statistics on those 1000 users

This would greatly reduce the scan time while remaining fairly accurate.