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.