Feature Request: Assigned different appenders to a category with different log levels

[stanleyxu2005]

Hi Project Maintainer.

How I implement now

Before using log4js, I use console to log everything. By using log4js, I want to print everything into out.log and in addition, print all errors into err.log.

After reading the document, I should do it as follows:

1. Define 3 appenders: out with type: 'stdout', err with type: 'stderr' and logErrOnly with type: 'logLevelFilter', logLevel: 'error'
2. Define a default category with 2 appenders out and logErrOnly.

That's what I found in the FAQ section (also see docs/logLevelFilter.md)

How I propose to implement

I'd like to know, if it is considered to be put into roadmap, to configure it is a straightfoward way like this

{
   appenders: {
       out: { type: 'stdout' },
       err: { type: 'stderr' },
   },
   categories: {
      default: { 
         appenders: [
            { name: 'out', logLevel: 'all' },
            { name: 'err', logLevel: 'error' }
        ]
      }
   }
}

[lamweili]

Pull requests are welcomed!