add severity level support for all linters

[domingo2000]

This PR fixes #348.

Introduce a severity level of :error by default to all the linters that does not have a configured severity level. Introducing a new severity level property in the linter configuration we achieve that now all the linters can have a severity property in the .erb-lint.yml.

Because currently the linter already has support for some compatibility with the severity levels of rubocop, this change only extends the same behaviour but for all linters.

This permits for example to omit the errors of MyCustomLinter when running erblint foo.erb with the following configuration

MyCustomLinter:
    enabled: true
    severity: info

This is useful to introduce linters in a codebease in an incremental manner, to configure to not throw errors when introducing a linter but still seeing it in the editor, for example when running the linter in CI.

[domingo2000]

@etiennebarrie Could you help with the revision here?

[etiennebarrie]

Can you add information about the feature in the README?

[domingo2000]

@etiennebarrie Done!

I created a section of the severity configuration in the README.md. Also improved the CLI documentation on the --fail-level option that makes this feature more usable on different severity values.

[domingo2000]

@andyw8 @etiennebarrie Any updates on this?

[meraioth]

@jenshenny @etiennebarrie any update? We are looking forward to this feat

[andyw8]

I'm not a maintainer of this repo, please stop pinging me.