Improvements to file logging

[Ziinc]

Currently, in master, the logs are stored in a non-optional manner.

I think a good improvement for this would be to make the file logging optional, as not everyone has log rotation configured or needs logging (especially ad-hoc users).

Mentioned first here: https://github.com/oltarasenko/crawly/pull/155#discussion_r544085821

Some improvements i can think of is:

• optional switch to enable file logging
• timestamps to log files (https://github.com/oltarasenko/crawly/pull/155#discussion_r544086672)
• opt-in dep for :logger_file_backend (https://github.com/oltarasenko/crawly/pull/155#discussion_r544092120)

[Ziinc]

#170 fixes point 1 and 2. Only making of file logger backend an opt in dep remains.