Add XDG configuration

Overview

I would like to propose using the the XDG specification for managing this gem's configuration which is a UNIX standard. Example:

# Global (default)
$HOME/.config/rubycritic/configuration.yml

# Local (alternative per project configuration -- Takes precedence over global configuration)
$HOME/projects/my_example/.config/rubycritic/configuration.yml

This can be easily solved by using the XDG gem as a dependency to this project. Even better, you can use the Runcom gem which is a superset of XDG for allowing users to have either a global and/or local configuration since XDG doesn't support local configurations by default.

Doing this work would resolve the following related issues:

• Add settings file.
• Add excluded paths/files.
• Reek configuration not being picked up.
• Configuration integrity - This one may or may not be resolved with this work but thought I'd mention it regardless.

Screenshots/Screencasts

Here's an example of ruby code used in the Rubysmith project to show how easy it would be to implement this:

💡 Associated source code can be found here.

Desired Behavior

By using Runcom, which is built on top of XDG, you'd get the following enhancements for free:

• Checks to see if local configuration exists and loads it.
• If local configuration doesn't exist, then it checks to see if global configuration exists and loads it.
• If global configuration doesn't exist, then default Rubycritic configuration is loaded instead.

By doing this work, we'd have a way to remove .rubycritic.yml from the root of everyone's projects.

Start by reading the referenced lib/rubysmith/cli/configuration.rb example and inspect how Rubycritic currently finds .rubycritic.yml. Verify the desired precedence of local configuration, global configuration, and defaults, then confirm that the root-level .rubycritic.yml can be removed without losing the related configuration behavior.