Additional values/columns derived from tool result

[PhilippWendler]

Factored out from #994:

Right now, BenchExec creates up to two derived values from the result of the tool: the result category (correct/wrong/etc.) and a score. Once we support more properties (cf. #996), it can be useful to derive more columns, e.g., an indication how close the actual and the expected result were. So it seems to generalize this part and let the Property (after #996) return an arbitrary amount of named values, that BenchExec would then take and treat like other values in the result, e.g., show columns for them.