Add an option to apply a threshold on the checkout

[rbouqueau]

The threshold could be computed from the actual checkout, not history. Possible command-line: git ls-tree --name-only -z -r HEAD|egrep -z -Z -E '\.(h|cpp|hpp|c)$' |xargs -0 -n1 git blame --line-porcelain|grep "^author "|sort|uniq -c|sort -nr

A default threshold could be 0.1% of the total lines. All the contributors below this point would be multiplied by 0.

At least we should add the checkout stats somewhere in the formula.