[FEATURE] Add option to disable check for hidden headers

By default, links to a content element with hidden header will not use a given human-readable fragment. This is because the intended use case of this extension are anchors on heading elements.

The new setting "checkForHiddenHeaders" allows to disable this check. The TYPO3 administrator or integrator is then responsible to provide a suitable Fluid layout for content elements that renders the "{fragmentIdentifier}" at a different HTML tag.