Remove ruby-enum dependency

[ojab]

Fixes #135

[ojab]

dunno what error message should I put here, but existing option ':' does not exist for CommonMarker::Config::Render looks wrong.

Overall I think that it could be just OPTS.fetch(type).fetch_values(*Array(option)).inject(0, :|) and that's it.
Right now we're overly strict in accepted types (why Set.new([:DEFAULT]) cannot be passed?) and IMHO it's expected that [] will be counted as a use defaults.

[gjtorikian]

Good points. Regarding being "overly strict"—I have a tendency to lean towards paranoia when interactions with C occur, hence the small whitelist of accepted types.

[ojab]

Sending to review due to First-time contributors need a maintainer to approve running workflows.. bundler exec rake passes locally.

[ojab]

Ugh, CI is failing due to

commonmarker.gemspec:24:29: C: Gemspec/RequiredRubyVersion: required_ruby_version (2.4, declared in commonmarker.gemspec) and TargetRubyVersion (2.5, which may be specified in .rubocop.yml) should be equal.
  s.required_ruby_version = ['>= 2.4.10', '< 4.0']
                            ^^^^^^^^^^^^^^^^^^^^^^

(rubocop dropped 2.4 support a while ago, so unfixable)

lib/commonmarker/node/inspect.rb:30:13: C: [Correctable] Style/RedundantBegin: Redundant begin block detected.
            begin
            ^^^^^

which is required and not redundant on 2.4.

Also specs are failing for 2.4 locally (KeyError does not have #key).
Since CI on 2.4 is not running — is it really supported or constraint in gemfile can be bumped to 2.5?

[gjtorikian]

Since CI on 2.4 is not running — is it really supported or constraint in gemfile can be bumped to 2.5?

Let's actually bring the constraint to 2.6, and remove 2.5 from CI. 2.5 is EOL and I don't want to support EOL Rubies. Thanks for bringing this up!

[ojab]

Done, first commit bumps ruby to 2.6, second actually removing ruby-enum. And again First-time contributors need a maintainer to approve running workflows..

[gjtorikian]

Thank you for your contribution here, I really appreciate it.