feat: configuration file of rules

[0xtekgrinder]

fixes #82

This PR adds a few things:

• A severity of issues and rules (info, warning and error)
• Parsing of a configuration file to add new rules, update severity of rules or remove default rules
• Appropriated documentation for configuration file and the new system when creating a rule
• A flag to init an empty configuration file

This refactor a lot of code so I am really open to any criticism as I don't believe it is the best way to implement these features.
I also added a data field in the rules to be used later on.

[notJoon]

Thank you for adding the feature you suggested recently. I believe this will become a very important functionality for linter.

However, I've left some comments that could be improved. Please check them when you have a chance. thanks!

[notJoon]

@0xtekgrinder
The basic severity types are currently uniformly set to SeverityError, but it seems that re-adjustment will be necessary later. This is because most of them are more about style rather than actual errors, so I believe the most of linter's initial value should be at a warning or lower level.

Anyway, most of all, it looks good to me. Thanks for the big contribution. 👍