new report keeps previous reports

[boly38]

Following #12

Goal is to provide a new report in addition to a previous one

• add "keep-history" option (doc) : done // this is just commit history, not related to the gh pages content
• need to checkout current gh-page state before running tests : add getPreviousReport.sh
• need to review and fix getPreviousReport.sh
  In this example user is doing the same. Check adding pushd $HOME
• validate to get 2 reports from different branch/datetime using manual build trigger
  • master build (push)
  • boly38 build (manual)
  • both results available at published gh pages