- added MongoDB collector, supporting core status and replication status
- added MySQL collector, supporting core status, master status, slave status,
slave hosts, and variables
- added PostgreSQL collector, supporting core status, replication status, and
settings
- added RabbitMQ collector, supporting core status and cluster status
- added Redis collector, supporting core status and cluster status
- reduced System collector payload size, deducting around 26% when using binary
- added `--conf` support, reading single-file collector configs in JSON and YAML
formats, or multi-file `conf.d/`-style directories
- made numerous refactorings and test improvements throughout, including
extending documentation extensively to provide examples of collector payloads