Properly close the writer and wait for all writes to finish

[adieu]

I use logger.Writer() as io.Writer for the output of my program and found that sometimes when the program exit too fast, some output wouldn't be displayed even if I close the writer before exit.

After reading writer.go I found that when we call writer.Close() we don't wait for entry.writerScanner() which runs in another goroutine to finish.

This commit adds WriterLevelWithClose which returns a closeFunc to properly close the writer and reader.

Eventually we might have to modify Writer and WriterLevel to return the closeFunc too which is not backward compatible or we might wrap the writer and return an io.WriteCloser instead.