Logging improvements

[richteer]

Right now, the log initialized in main.py always spews to stdout. There should be some way either by configuration or via the command line to specify a file for the log to write to instead (particularly useful if running interactively (python3 -i)).

Also, the log is stuck at a hardcoded level. It should also be configurable what level of detail should be outputted (DEBUG vs INFO, etc.)

Examples of the usage:

python3 main.py --logfile foo.log --loglevel DEBUG

...or in config.json:

{
    "logfile": "foo.log",
    "loglevel": "DEBUG",
...
}

[richteer]

This could follow a solution to #57, where config.json should be loaded in main.py before the logger is initialized.

[richteer]

Implemented in PR #63