Support log output to multiple sources

[nhgrif]

From your README's TODO list:

print to a text file when used on a device

But competing logging libraries allow me to add different loggers. As an example:

    /**
     Adds CocoaLumberjack loggers to the project.
     */
    static func setUpLogging() {
        #if DEBUG
            setenv("XcodeColors", "YES", 0)
            DDLog.addLogger(xcodeConsoleLogger)
        #endif
        DDLog.addLogger(fileLogger)
        DDLog.addLogger(appleSystemLogger)
    }

I think in an ideal world, I'd be able to not only do this, but also specify the minimum logging level for each logger I attach. So for example, the Xcode console gets all log levels, but the file logger only gets debug and up, and the Apple system logger only gets warning and error.

[nhgrif]

As an additional note, these loggers we add in the above example can actually specify their custom formatter and organize the different parts of the log message however they want. This should be supported.