[log] Support log to file

[at15]

Currently stdout is the only output, I don't think it even writes any thing to stderr, though 12 factors strongly discourage log to file and suggest using redirect, but I find logging to file with default naming is pretty useful, especially when you log a lot and you can't scroll up the terminal any more.

• log to file
• add flush to improve performance [log] Refactor and measure performance using benchmark #90
  • need to call it when the got SIGTERM
  • what if both the application and the library try to trap SIGTERM, then which one will work
• what's the benefits of logging to both stderr and stdout
• rolling log, log rotate
  • https://github.com/natefinch/lumberjack
  • I think it's better to use external program, though we could have this built in for easier deployment

[at15]

just use a file as io.Writer and pass to handlers, it's already implemented long time ago

add Flush on Logger maybe we should have a FlushAll due to tree of logger #33

log rotate is not supported, might put that in logx