Detect missing readme headers

[ernilambar]

Related to https://github.com/WordPress/plugin-check/issues/402

• Added check_headers() method in Plugin_Readme_Check
• Add separate private method for getting ignored filters get_ignored_warnings()
• Two headers are checked here:
  • Tested up to
  • Contributors
• New method will check if those fields are empty
• Checking for the field is enabled/disabled based on wp_plugin_check_ignored_readme_warnings
• Since contributor_ignored is returned from the above filter, detecting of Contributors is disabled.

If the PR is acceptable then I will add tests for the changes and update other tests accordingly. Other tests also need to be updated because currently we have assumed there will be no warning for missing Tested up to field.