Implement the feature to log to file, with support for JSON. (#234)

This allows pgcopydb to log both to the terminal and also to a file, where
the format is not adjusted for terminal width (long time format, source file
and line always included).

Also implements logging in JSON, using a JSON object per line with a static
set of attributes that includes all the metadata we have about the log
message.

The control of this feature set is entirely based on using environment
variables, no new command line switch has been introduced. It is possible to
log both in plain text with colors on the terminal and to a JSON file at the
same time.