Allow custom options for URL normalisation

[falkecarlsen]

Note that the www domain label is enough to change the equality of two URLs according to the Url-package. This is expected behaviour, as it does actually change semantics.

For scraping, some links may use the domain label and others may not, e.g. human-written links would probably omit the label while automated links would probably include it for completeness. This could result in a page with two seemingly different tasks, that are actually pointing to the same page.