[ADD] XQuery, ft:search: new options added. Closes #762

* Documentation:
  http://docs.basex.org/wiki/Full-Text_Module#ft:search

* Example:

  ft:search("db", ("A", "B"), {
    "mode": "all words",
    "distance": {
      "max": "5",
      "unit": "words"
    }
  })