Allow logging to be overridden

[robstrong]

I agree with #2 and #3 that logging should not forced upon users of this library. But I also agree with @rcrowley that the logging is useful and needed. This PR doesn't change the default logging functionality but adds a Logger var on the package that can be set to override where log messages go.

This can be useful for applications that already have a logger setup and want the goagain logs to go to the same place. You just set

myLogger := log.New(myWriter, "err: ", log.LstdFlags)
goagain.Logger = myLogger

Or if you wanted to disable logging altogether, you can set

goagain.Logger = nil

[robstrong]

@rcrowley Any thoughts on this?

[brasic]

Would love it if this could be merged. The current approach makes things difficult for applications that use specially formatted logs parsed by tools like logstash.

This is similar to a hardcoded logger that was changed to be configurable in the standard library's httputil/reverseproxy: https://github.com/golang/go/commit/1a1d43239e42184694d0207ffad9bbb77d8e8f76