Configuration defaults

[rizsotto]

This ticket is about the rust code.

The new configuration of the Bear program is more complex than before. It has module documentation (and probably will have man page too) to explain the possibilities to users. But the configuration is optional. If the configuration file is not given, the program is using sane default values.

The challenges I would like to solve:

• How users will know what the default configuration is?
• How can they quickly have a configuration file?

Ideas to evaluate as possible fix for the problem:

• the --help-config shows the current defaults. And it shows in YAML format (so it's easy to use as a starting point)
• the project deploys a default config file as template (with a lot of comments included)