ValExtLinks to-do list

- Add argument --only-200-ok which doesn't treat the other entries in my OK_CODES array as OK.  This may allow us to catch additional problems.

- Audit exceptions list so it doesn't acquire cruft: mention URLs in list that aren't in extlinks.csv, and URLs that were found but returned different codes (or were OK instead of NG).  This can be done with a parallel array of 0/1s that track whether each exception in the exceptions list was used.

- Automatically highlight pages that do not have archive.org or archive.is snapshots (see here for the archive.is API).  When checking for snapshots, we shouldn't just ask archive.org if it has one.

- OneDrive links like http://cid-639aa31296681bfe.skydrive.live.com/self.aspx/Oni/Images/head.png are 404s but return 200.  Need to detect these!