Revert "Changed flow numbering in -F option (was: 0,1,2,... - now: 1,2,3,...)"

Patch was a bad idea since it breaks on one hand the backward compatibility and
on the other hand it was not complete (the output was not changed
accordingly)
Document that -F -1 refers all flows
Merge pull request #87 from alexander-zimmermann/next

Revert patch 8deded9adf