Retrieve the image metadata [for future use].

Ugh, this is so ugly.  I really hope this isn't the best way.  But it's
the *only* way I've been able to find.  It boggles my mind that NSImage
doesn't have a -(NSDictionary*)metadata property or somesuch.
One way of specifying filters.  An ugly way.

I thought it'd be wiser to just make use of an existing simple language
and parser, but unfortunately the most promising seemed to be old-style
property lists, but dealing with them is just a pain.

Committing this for posterity in case I ever want to be reminded how
obnoxious this was, before I go off shortly and rewrite it to be way
simpler.
Way simpler way of parsing filter parameters.
Expanded on the error message for updateMovieFile failing.

Based on experience.  Sigh.
Implemented the actual filtering.  Rudimentary, but it works.
Added the --dryrun flag.

Useful for checking that what you're trying to filter out is indeed
being filtered out.