Change score calculation from passed/total

[Felixoid]

Hello. I've found it a little bit inconvenient to see that skipped and duplicated mutations decrease the scores. https://github.com/zimmski/go-mutesting/blob/master/cmd/go-mutesting/main.go#L154

IMHO, the score should be calculated as in 7ccb9f3, so success / (success + failed). Skipped and duplicated should be still in totals, but not affect the scores.

In other words, I strictly disagree with the following calculation:

The mutation score is 0.586957 (27 passed, 0 failed, 7 duplicated, 19 skipped, total is 46)
The mutation score is 0.466667 (7 passed, 0 failed, 2 duplicated, 8 skipped, total is 15)