Add options to scale users baed on the amount of commits they did

New Options

• --user-scale-by-commits - Enable commit-based scaling
• --persist-user-scale - Users stay at their max size instead of reseting when reappering
• --commit-scale-type TYPE - How scaling grows: log, linear, sqrt, or exp
• --commit-scale-factor N - Amplify the scaling effect
• --user-min-scale N / --user-max-scale N - Clamp the size range

Example:

./gource --user-scale-by-commits --commit-scale-type log --auto-skip-seconds 1.1 -s 0.1 --highlight-users --commit-scale-factor 2.0 .