Implement HistogramReporter

Fixes #81
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Extract and test Histogram logic
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Add 300s status code handling for metrics test
Mark 300s as success when --redirects=-1

refs #93, #88
Rename MarkRedirectsAsSuccess to NoFollow
Inline condition for checking NoFollow
Add comment back for ignoring redirects when NoFollow
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Vegeta dump command

Fixes #95
Update documentation