Write a variadic function ElementsByTagName that,
given an HTML node tree and zero or more names,
returns all the elements that match one of those names.
Here are two example calls:
func ElementsByTagName(doc *html.Node, name ...string) []*html.Node
images := ElementsByTagName(doc, "img")
headings := ElementsByTagName(doc, "h1", "h2", "h3", "h4")