Optimize flow anlysis (~25% faster). Report count of skipped lines, if any.

Using dumbench (https://metacpan.org/module/Dumbbench) to run flamegraph
on a file with ~140,000 lines:
old: Rounded run time per iteration: 3.472e+00 +/- 1.7e-02 (0.5%)
new: Rounded run time per iteration: 2.618e+00 +/- 1.3e-02 (0.5%)
Merge pull request #10 from timbunce/optimize

Optimize flow analysis (~25% faster). Report count of skipped lines, if any.