Enhancement request: config file option log_format = "json"

This configuration option aligns with the same one in [Consul](https://developer.hashicorp.com/consul/docs/agent/config/config-files#log_format), [Nomad](https://developer.hashicorp.com/nomad/docs/configuration#log_format), [Vault](https://developer.hashicorp.com/vault/docs/configuration#log_format).
The value here is that logging output will be readily ingestible into metrics systems so that we can keep an eye on all instances even as they are dynamically added and removed.

The issue names no files, tests, or entry points, so first trace how consul-template reads configuration and emits logs. Compare the requested log_format option with the linked Consul, Nomad, and Vault behavior; done means JSON logging is configurable and produces output suitable for ingestion by metrics systems.

Written by the indexing model from the issue text.