Added feature:

 - Discard Old Builds
with the syntax:
job:
    discard: # Discard old builds after:
      days: 1 # Optional, number of days after which the build is deleted
      number: 2 # Optional, number of builds after which the build is deleted
      artifact_days: 3 # Optional, number of days after which the artifact is deleted
      artifact_number: 4 # Optional, number of builds after which the artifact is deleted