Add render command

[endorama]

#46 adds an additional changelog rendering functionality to the application.

This functionality should be exposed as a render command.

The render command should run the changelog.Renderer Render() method to render the consolidated changelog for a version in an Asciidoc file. To do this it will need a --version flag, search for the consolidated changelog (<version>.yaml in changelog_destination folder) and use it to fill the template. It should output the rendered Asciidoc file in rendered_changelog_destination (this setting should be added to internal/settings package).