output for validate is printed with yellow warning prefix

[MrBartusek]

As an example take that code:

config.validate({ allowed: 'warn', output: Logger.warn });

I redirect all logs from convict to my logger (feature added in #301). The messages are something like
warning: something broke and a warning is in yellow text. The problems are that most of the loggers add own prefix like [WARNING] or something so result in messages are [WARNING] Warning: something broke (which looks funny)

The best solution will be to don't add that prefix by default