gate initialization with traits behind option

[hankim813]

https://segment.atlassian.net/browse/INT-592

This puts the feature introduced in #20 behind an option. This means that people would need to opt in in order to initialize facebook pixel with existing traits that's been .identify()ed already.

• [ ] docs: https://github.com/segmentio/site-docs/pull/1797
• [ ] update metadata
• [ ] mongo script to set initWithExistingTraits to true for all existing user: https://github.com/segmentio/mongo-scripts/pull/209

@thehydroimpulse @wcjohnson11

[wcjohnson11]

lgtm