fix(quant): corpus-line option, output improvements

[M4tteoP]

Closes #391

• fixes corpus-line option not working
• Adds a json output even if there have not been false positives detected (before empty output):

{"count":1,"falsePositives":0,"falsePositivesPerRule":{},"skipped":0,"totalTimeSeconds":0.088605792}

• Adds some details about the run even if there have not been false positives detected:

Before:

No false positives detected with the passed corpus

After:

Run 1 payloads (1 skipped) in 86.454458ms
No false positives detected with the passed corpus

• Adds the skipped counter to make the output clearer on some complex combinations of options.

# In this case I'm running just up to 1 line, and I also ask to run just the 10th line.
# The output tells me that 1 payload have been evaluated and it has been also skipped
▶ go run main.go quantitative --crs-path ~/Repo/coreruleset --corpus-size "10k" -n=10 --lines 1
11:22PM INF ⏳Running quantitative tests with 10 goroutines
Run 1 payloads (1 skipped) in 92.97825ms
No false positives detected with the passed corpus

# When a specific line is required, it is now explicit that all the other payloads have been skipped.
▶ go run main.go quantitative --crs-path ~/Repo/coreruleset --corpus-size "10k" -n=10 --lines 100
11:23PM INF ⏳Running quantitative tests with 10 goroutines
Run 100 payloads (99 skipped) in 99.074584ms
No false positives detected with the passed corpus

[fzipi]

LGTM!