Support log output in json

[lox]

This adds a --log-format option to buildkite-agent start that allows for json output for better integration with structured logging systems.

$ buildkite-agent start --debug --log-format=json
{"ts":"2019-03-31T22:51:28+11:00","level":"NOTICE","msg":"Starting buildkite-agent v3.10.2 with PID: 3282"}
{"ts":"2019-03-31T22:51:28+11:00","level":"NOTICE","msg":"The agent source code can be found here: https://github.com/buildkite/agent"}
{"ts":"2019-03-31T22:51:28+11:00","level":"NOTICE","msg":"For questions and support, email us at: [email protected]"}
{"ts":"2019-03-31T22:51:28+11:00","level":"INFO","msg":"Configuration loaded from: /usr/local/etc/buildkite-agent/buildkite-agent.cfg"}
{"ts":"2019-03-31T22:51:28+11:00","level":"DEBUG","msg":"Bootstrap command: buildkite-agent bootstrap"}
{"ts":"2019-03-31T22:51:28+11:00","level":"DEBUG","msg":"Build path: /usr/local/var/buildkite-agent/builds"}
{"ts":"2019-03-31T22:51:28+11:00","level":"DEBUG","msg":"Hooks directory: /usr/local/etc/buildkite-agent/hooks"}
{"ts":"2019-03-31T22:51:28+11:00","level":"DEBUG","msg":"Plugins directory: /usr/local/var/buildkite-agent/plugins"}
{"ts":"2019-03-31T22:51:28+11:00","level":"INFO","msg":"Registering agent with Buildkite..."}
{"ts":"2019-03-31T22:51:28+11:00","level":"DEBUG","msg":"POST https://agent.buildkite.com/v3/register"}

This lays the groundwork for structured field logging too, although the only field currently implemented is the agent name.

[lox]

This is what things look like for normal console logging:

This is what json logging looks like: