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Logger Cleanup

The buffered writer is faster, but we added a bug! It must be flushed (written to disk) before the program exits, or any pending log messages will be lost!

It's also common to log messages across the network in some scenarios, and those kinds of loggers will also need to be flushed before exit – so we should take that into account in our implementation.

Assignment

Clean up logger resources before exit.

As you create your logger, also create a "close" function that cleans up any logger resources.
For the file logger, that close function should .Flush the buffered writer and .Close the file. One valid signature is:
```
type closeFunc func() error

func initializeLogger(logFile string) (*log.Logger, closeFunc, error)
```
For the STDERR logger, return a no-op close function that returns nil.
Call the close function before Linko exits. Don't defer it directly (you'd lose the returned error). Instead, defer a wrapper that calls it and prints any cleanup error to STDERR.

Once again, we resort to writing directly to os.Stderr – the logger isn't in a usable state at this point.
Start your server with LINKO_LOG_FILE=linko.access.log set:

LINKO_LOG_FILE=linko.access.log go run .

Run and submit the CLI tests from the root of the Linko repo.