Add a command-line flag to the mutation command to support killmap computation

[rjust]

In Defects4J, Major is configured to determine all killed mutants and to compute the mutation score; Major does not compute a killmap by default. Computing a killmap, however, is required for a number of research projects.

The current solution for enabling the killmap computation is to adapt the defects4j.build.xml.

The defects4j mutation command should support a new flag (-K) that allows a user to easily compute a killmap without changing the build file.