ignore forks scanning a user

[marcorosa]

When scanning all the repositories of a user, add a parameter to the method (e.g., forks=False|True) to decide whether to scan or not the forks of that user (i.e., the repositories that the user forked and that appear in her repo list).

Rationale: usually, the forks are outdated and, if there is any commit by the user, these commits are generally few and never contain credentials

[marcorosa]

Solved by #33