Enable control of the log level via environment variables

[gajus]

It would be useful to be enable control-override of the createLogger level option.

This setting ought to be controlled using environment variables, e.g. BUNYAN_LOG_LEVEL. This way it would be possible to enable different log levels without tracing down every instance of createLogger or leaving it up to the developer to respect the suggested BUNYAN_LOG_LEVEL setting.

[trentm]

See #176 for a similar issue. IMO control of logging output streams and levels belongs in the application using Bunyan, and not in the library itself. In #176 I created an example file that shows how the app using Bunyan can control this with environment variables itself: https://github.com/trentm/node-bunyan/blob/master/examples/mute-by-envvars-stream.js

Similar could be done for the log level. Something like:

var level = process.env.BUNYAN_LOG_LEVEL || 'info';
var log = bunyan.createLogger({name: 'foo', level: level});

[gajus]

The problem with:

var level = process.env.BUNYAN_LOG_LEVEL || 'info';
var log = bunyan.createLogger({name: 'foo', level: level});

is that you then leave it up to the developers to enforce a consistent standard. Which is fine. On the other hand, modules such as debug focus on standardising conventions, which ... at the level of debugging libraries, has been tremendously useful.