Add stderrLevels option to Console Transport and update docs.

[paulhroth]

This PR lets users choose which levels to log to stderr instead of stdout. It's non-breaking and fully backwards compatible with previous options and behavior. I hope you like it 👍

The new Console Transport option is stderrLevels, which accepts an Array of strings containing the levels to log to stderr instead of stdout.

The code is thoroughly unit tested, contains input validation, and converts the Array provided to options.stderrLevels to an Object for faster key-lookup times. The README.md and docs/transports.md files have also been updated to better reflect the current state of the Console Transport.

Example usage:
Create a Console that logs 'error' and 'warn' levels to stderr, and other levels to stdout:
var console = new (winston.transports.Console)({ stderrLevels: ['error', 'warn'] });

Create a Console that logs nothing to stderr and everything to stdout:
var console = new (winston.transports.Console)({ stderrLevels: [] });

Create a Console that logs certain custom levels to stderr:

var console = new (winston.transports.Console)({ 
  levels: myCustomLevels,
  stderrLevels: ['delta', 'epsilon']
});

Backwards Compatibility
The current state of the Console has it logging these levels to stderr:

• error and debug (if options.debugStdout is false)
• error (if options.debugStdout is true)

This behavior is kept as the default for stderrLevels in order to ensure this change is non-breaking: stderrLevels defaults to the equivalent of (options.debugStdout ? ['error'] : ['error', 'debug']). This means that everyone not using the new stderrLevels option will see no change in the Console Transport's behavior.

Note: Another PR from July 2013 exists for the same feature (#286), but caused breaking changes and was never merged. Hopefully since this new PR is non-breaking and takes the debugStdout option into consideration, uses an Object for fast key-lookup times, includes unit tests, input validation and updated documentation, the feature we both want to see can finally get in in 2015 :)

[DominiqueLeroux]

Nice improvement! I also have been wanting to control level-to-output stream mapping for a while.

[ThibodeauJF]

Let's use that improvement the world needs it 👍

[yoannmoinet]

This seems to be a valuable improvement.

[indexzero]

An immediately invoked function expression is a rather cumbersome way to do this. Could you clean it up a bit?

[indexzero]

I am +1 on this if we can cleanup the obtuse syntax a bit. There has to be a simpler way to setup the initial state.

[paulhroth]

@indexzero Thanks for the code review, I made some changes following your advice. How does this look? Should I move the setStderrLevels function out of the Console constructor (right now it's there so as to follow the helper-function pattern seen with throwIf in the other transports, e.g. in daily-rotate-file.js line 31)?

[indexzero]

This is good. Especially the deprecation notice. This will land in 1.1 and we will fully remove debugStdout in 2.0.0

[indexzero]

This was cherry-picked.