Release v1.12.0 (uber-go#1045)

This release v1.20.0 of Zap with a couple new features for customizing
the JSON encoder. Namely,

- support skipping newlines between log statements (uber-go#989)
- support changing the reflection JSON encoder (uber-go#1039)

Refs GO-1085

Release v1.19.0 (uber-go#987)

Release v1.18.1, fix panic

This releases a fix for the NopLogger breakage accidentally introduced
in v1.18.0.

Prepare release v1.15.0 (uber-go#819)