Ref field support

[crystoline]

Description

PHPOffice/PHPWord currently does not support adding the field of type REF.
I would like to add a field to reference a bookmark

Fixes # (issue)

• Add REF options to $fieldsArray in PhpOffice\PhpWord\Element\Field.php
• Added writeRef method to Handle REF options

$textRun->addField('REF', [
      'name' => 'my-bookmark'
  ], [
      'InsertParagraphNumberRelativeContext',
      'CreateHyperLink',
  ]);

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[waltertamboer]

@PowerKiKi Would you be willing to review this PR? That would be much appreciated 👍

[coveralls]

coverage: 96.658% (-0.6%) from 97.217%
when pulling 195a326 on crystoline:ref-field-support
into 8b891bb on PHPOffice:master.