Add YAML formatter #463

[lazebny]

There is a basic implementation of YAML format in a ready-to-work state.
A way for improvement is to abstract YAML encoder. Since there is no standard behavior for such encoder that requires additional invesigation.

[zorbash]

Thank you @lazebny just a few remaining comments and we can merge this.

[lazebny]

Thank you @lazebny just a few remaining comments and we can merge this.

Thank you @zorbash for the review! Would you be so kind as to release the new version to hex after merging?

[lazebny]

@zorbash any updates?

[zorbash]

@lazebny One last thing. We have to update the README here to mention the new task which exports a YAML spec. We should also mention in the readme that {:ymlr, "~> 2.0"} has to be added to the mix file since it's marked as an optional dependency in the mix file of open_api_spex.

If a user doesn't add the yml dependency, they'll see the following error which isn't particularly friendly.

** (UndefinedFunctionError) function nil.encode/2 is undefined
    (open_api_spex 3.11.0) lib/mix/tasks/openapi.spec.yaml.ex:31: Mix.Tasks.Openapi.Spec.Yaml.encode/2
    (open_api_spex 3.11.0) lib/open_api_spex/export_spec.ex:18: OpenApiSpex.ExportSpec.call/3
    (mix 1.13.3) lib/mix/task.ex:397: anonymous fn/3 in Mix.Task.run_task/3
    (mix 1.13.3) lib/mix/cli.ex:84: Mix.CLI.run_task/2
    (elixir 1.13.3) lib/code.ex:1183: Code.require_file/2

[lazebny]

@lazebny One last thing. We have to update the README here to mention the new task which exports a YAML spec. We should also mention in the readme that {:ymlr, "~> 2.0"} has to be added to the mix file since it's marked as an optional dependency in the mix file of open_api_spex.

If a user doesn't add the yml dependency, they'll see the following error which isn't particularly friendly.

** (UndefinedFunctionError) function nil.encode/2 is undefined
    (open_api_spex 3.11.0) lib/mix/tasks/openapi.spec.yaml.ex:31: Mix.Tasks.Openapi.Spec.Yaml.encode/2
    (open_api_spex 3.11.0) lib/open_api_spex/export_spec.ex:18: OpenApiSpex.ExportSpec.call/3
    (mix 1.13.3) lib/mix/task.ex:397: anonymous fn/3 in Mix.Task.run_task/3
    (mix 1.13.3) lib/mix/cli.ex:84: Mix.CLI.run_task/2
    (elixir 1.13.3) lib/code.ex:1183: Code.require_file/2

@zorbash updated README.md, please re-check

[zorbash]

Thank you @lazebny 👍