Adding support for time-based Log Rotation

In my project there was a requirement to rotate logs at 4:00 AM every morning. As you know, It is a common use-case to rotate the logs. So I implemented the same and committing it back to repository. Please go through the changes I've made and see if it can be merged along with your master (w/ or w/o modifications).

I have modified the Readme & also added an example of how to make use of this feature at: https://github.com/thakkar-rushikesh/log.js/blob/master/examples/file-rotate.js.

Please review and provide your suggestions if any.