Make log & warn functions replaceable

[alanpearce]

By allowing overrides of the functions used for logging, we can allow the output from this middleware to be processed according to the logging library used in the application.

[SpaceK33z]

There is also a reporter option, that does about the same. There is only one console.log that is not inside this reporter. Could you maybe move that console.log over there instead?

[alanpearce]

Well, the reason I didn't touch reporter is because it seems non-trivial to replace as a library user, and then relies on the API not breaking over time. Also, from the name, I assumed it would be a function run at the end of a compilation to format the output. IMO it should not even be used to log that the bundle is invalid, just to report the stats (or not) from a compilation.

I could do it, but it seems incorrect to me. I'd have to add some other state value or some other decision flag for the other console.log, and then what happens if maybe more logging is added later? More state values? Would it really be desirable to maintain a most-likely increasing number of switch cases rather than adding a log line, which could then be logged in a way that fits into whatever framework or log library the user may be using?

If you really think it should be done like this, please let me know what you'd like to have as the state value for the last log call outside of the reporter, or how to otherwise decide if this is a "waiting" event.

[SpaceK33z]

Okay, fair enough. We can let the reporter method only be for processing the output.

Could you add a test for this and fix the broken tests? The relevant tests are in test/Reporter.test.js.

[SpaceK33z]

Looks good.

[SpaceK33z]

Thanks!