Logging initialization improvement

[ImplOfAnImpl]

Here I replaced init_logging_to and init_logging_with_env_var with a single function init_logging_generic that allows to specify an additional destination with separate filter and style settings.

The "log_file" is now passed to init_logging_generic as this second destination. The level is "info" to keep the log size reasonable, though we may change it to debug too (but note that the simple "debug" will be too noisy for node-gui due to 3rd-party crates, so we'll need to filter them out)

With this approach we

1. can have both the console and file log at the same time;
2. can be sure that the user's messing with RUST_LOG won't affect the log file.