Support logging to file

[markphelps]

Is your feature request related to a problem? Please describe.
Currently logs only go to STDOUT, would be great to support logging to the filesystem

Describe the solution you'd like
Support logging to file in configs

[aaronraff]

I am very new to this project but if you are looking for someone to lend a hand on either this issue or some other one, I would be happy to help out.

[markphelps]

Awesome. That would be great @aaronraff !

I was thinking it could be implemented somewhat like this: sirupsen/logrus#156 (comment)

With adding a field to the config: https://github.com/markphelps/flipt/blob/3430f47942167ce16a1e0fc6dd6272bd835584e4/config/local.yml#L1

Something like:

log:
  level:
  file: filename.log

I guess if file was empty or didn't exist it would just log to STDOUT like it currently does

[markphelps]

@all-contributors please add @aaronraff for code

[allcontributors]

@markphelps

I've put up a pull request to add @aaronraff! 🎉