[FEATURE] Introduce mandatory tag attributes

[ohader]

Introduces new Behavior\Attr::MANDATORY flag, declaring an attribute must be given for a particular tag to be valid.

(new Behavior\Tag('script'))
  ->addAttrs(
    (new Behavior\Attr('type', Behavior\Attr::MANDATORY))
      ->addValues(new Behavior\DatasetAttrValue('application/ld+json'))
)

The example above allows <script type="application/ld+json">...</script> but disallows all other occurrences not having type="application/ld+json", thus <script> or <script type="application/javascript"> still would be invalid.

Related: #70, #71