multiple AB testing support

[shoota]

TL;DR

should be able to send multiple event as AB testing experiment in plugins.

specs:

• control send flag per called getVariation queue command.

• auto reset send flag. (support SPA)

• target plugins

  • [x] vwo
  • [x] optimizely X

• testing

  • [ ] check tracking request with E2E testing

[shoota]

not finish e2e test implement.