DailyRotateFile transport: availability to prepend formatted date to log name

[ArnauAregall]

Hello,

I'm using winston in a project where is preferable that log files had the rolling date prepended instead of appended to the log name, so I added an option (prepend) to be available to configure this (the formatted date-time is still appended by default).

I hope you find it useful!

[indexzero]

Please conform to https://github.com/indexzero/styleguide

[ArnauAregall]

Didn't see the trailing whitespaces in my editor, just removed them.
@indexzero is the code conforming to the mentioned style guide?

[indexzero]

It is now, but may conflict with #505 so we need to investigate.

[indexzero]

Thank you for your contribution As of winston@2.0.0 this transport is now a stand-alone module: winston-daily-rotate-file. Please reopen this PR. We have opened a tracking issue there to ensure it is reviewed appropriately.