Make it possible to pass some information from the parent page

[nietaki]

Currently each of the pages are considered on their own. This isn't a flexible enough model, when some page's data is important in the context of the parent page - for example you're crawling some website to download all the images it links to, but want to know the anchor texts for each of them.

The solution would be to have ParserLogic.extract_uris() return {tag :: term, uri :: URI.t} | URI.t, where the latter would be converted to {nil, uri} and have it passed as an aditional argument to ParserLogic.parse(). The user could then choose to (or not to) include the parent tag in the parse_result for their convenience.

This will require some manual labour, but shouldn't be too bad ;)