Add log rotation

[victorhooi]

The logfiles can get large, especially if there's a lot of fingerprints and/or -debug has been turned on. (I'm thinking of main.stdout).

It may be worth looking into automatically rotating this.

Maybe something like lumberjack? Seems like an easy swap.

Alternatively, if we want to push this onto the user, we should mention this explicitly in README.md.

[schollz]

Seelog does this: https://github.com/cihub/seelog/wiki/Receiver-reference#rolling-file-writer-or-rotation-file-writer

This will be easy to implement.