Implement htmlParser.element.find() method

[Piotrek Koszuliński]

It is hard to find matching elements in htmlParser.element. Real DOM has couple of methods like find, findOne, byId, byClass, etc., but HMTL parser has nothing except forEach() which allows you to iterate over the entire sub tree and find whatever you need.

Implementing find/findOne which would work in the same way as in real DOM is reasonable, because the CSS selectors implementation would be huge. Therefore, I propose a simpler method accepting a callback or element name. It can be implemented in few lines of code using forEach().

Based on #12854.