[WIP] Add LxmlLinkExtractor class similar to SgmlLinkExtractor (#528)

I had to change the Selector HTML parse from `lxml.etree.HTMLParser` to `lxml.html.HTMLParser` to have those helpful `.make_links_absolute()` and `.iterlinks()`

There's still margin for some factorisation and cleanup (as much of the code is bluntly copied from `SgmlLinkExtractor`

implementation of #528 
looks like there is no need for nesting two "if"s, it can be reduced to `if self.scan_tag(el.tag) and self.scan_attr(attr)`