bench: added cli command -S to benchmark multiple files separately

Currently, all files are joined by default,
they are compressed separately but benchmarked together,
providing a single final result.

Benchmarking files separately make it possible to accurately measure difference for each file.
This is expected to be useful while tuning optimal parser.

programs/bench.h

@@ -25,6 +25,7 @@ void BMK_setBlockSize(size_t blockSize);
 void BMK_setNbThreads(unsigned nbThreads);
 void BMK_setRealTime(unsigned priority);
 void BMK_setNotificationLevel(unsigned level);
+void BMK_setSeparateFiles(unsigned separate);
 void BMK_setAdditionalParam(int additionalParam);
 void BMK_setDecodeOnlyMode(unsigned decodeFlag);
 void BMK_setLdmFlag(unsigned ldmFlag);