add support for vwo revenue tracking

[ndhoule]

From @DirtyAnalytics on May 22, 2014 18:53

http://visualwebsiteoptimizer.com/knowledge/how-add-revenue-tracking-tests/

Copied from original issue: segmentio/analytics.js-integrations#160

[hankim813]

this is supported here https://github.com/segment-integrations/analytics.js-integration-visual-website-optimizer/blob/master/lib/index.js#L61