add option to select the column-identifier for sorting.
add option to select the column-identifier for sorting.

IljaZakharov/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,sosy-lab/benchexec,dbeyer/benchexec,IljaZakharov/benchexec,sosy-lab/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec