Add methods to Source::URL for determining whether a URL is an image
URL, a page URL, or a profile URL.
Also add more source URL tests and fix various URL parsing bugs.
When the artist name couldn't found for a Newgrounds URL, for example
for `https://www.newgrounds.com/dump/item`, then the `profile_url`
method erroneously returned `https://.newgrounds.com`. This led to an
error later on when the artist finder tried to parse the invalid URL.
Also fix `strategy_should_work` to test that the profile URL is a valid
URL, and not to try to download the file when image_urls is empty.
Sometimes when a Newgrounds post is part of a set, there is a list of
links to other posts in the set in the artist's commentary. Exclude
these links because they're not really part of the commentary.
Example: https://www.newgrounds.com/art/view/boxofwant/annie-hughes-1 (NSFW)
