Refactor filtering and mapping for url extraction

We make a few changes to the URLs we extract such as filtering out
URLs that don't match a certain predicate and adjusting the values
that we hold.

This change converts these operations to use a common set of
`filterURLs` and `mapURLs` functions that can be passed a function to
call for each item in the array of *url.URL elements.
Updated in b526ef5. New tip is 2bebdef.