Multivariate feature flag and experiments support

[liyiy]

Changes

Multivariate support

• swap to v2 endpoint
• add getFeatureFlag method
• capture's send feature flags option for experiments
• example.js file to test with

Checklist

• [ ] Tests for new code (if applicable)
• [ ] TypeScript definitions (module.d.ts) updated and in sync with library exports (if applicable)