Introduce a createLevel call, document, add tests.

[aahoughton]

This is a -- possibly stopgap -- solution to the need for levels beyond the default set. It's imperfect, but it's also self-contained and easy to understand.

Caveats:

1. There's no way to dynamically add dtrace probes after-the-fact of logger creation, so there's no way to trace levels in loggers that were instantiated prior to the level creation
2. I think this is completely safe in-re: Logger fields; we may be adding functions to the prototype left-and-right, but they'll be shadowed by the actual Logger instance, and we won't overwrite anything that exists
3. I'm a little uncomfortable with all the hand-holding / don't-shoot-yourself-in-the-foot going on in the createLogger call, but it's certainly safer ...
4. The documentation could be better. :/

.. thoughts?

[aahoughton]

Let me add, I'll be happy to update the CLI tool to take mappings -- something like a '--level' arg that lets you (someone) do something like '--level "notice=35" --level "foo=46", etc. ... but only if there's a sense that this work is headed in the right direction.

[aahoughton]

.. aaand I needed it anyway.

[ORESoftware]

+1

[siimon]

any possibility to get this merged? would be nice to have custom levels!

[ScheerMT]

I like the usage for adding a new log level in this PR. Came looking to see if bunyan had this feature already and surprised it didn't.

What else is needed to get this landed? My use case would include using this to keep all logging in one place and keep my custom stream logic in one place for forarding on the log messages to an external db.

[aahoughton]

@trentm anything I can do here to help? I'd like to stop depending on a forked repo. I'd be happy to get this based off the head of the tree, but are there other issues you'd like to see addressed?

[aahoughton]

Closing, rebasing, etc.

[aahoughton]

Migrated here, for anyone following along at home: #465