<select> element implementation

The introduction of <select> validation is released in v0.3.0.

The following rules apply:

• If a <select> element is submitted, its available <option> elements are inspected.
• If the required attribute is present, a value must be sent.
• Only values available within <option> elements are considered valid.
• <option> elements can have a value attribute to indicate their value, otherwise their text content will be used.