Add extraction for release dates for the new-format (at least) ninjatune xls files

Mostly putting up for review for the sake of this being a way to have people able to question my stuff and learn about the codebase.

This parses out the release dates in the newer of the two ninjatune xls formats (the old one has strings, as I recall, and I'm not sure it splits by country -- I haven't checked yet).
I'm actually not sure that this needs to be a lambda, now that you point it out. I'll have to try removing that.

Each part of the path is turned into a PathPart instance at some point, yes, because the PathPart instances are what implement the actual functionality to insert into a structure. There's actually a normalization process that takes what's here, turns it into a list of PathParts, and then turns it *back* -- this is so that it can be filled in with defaults that aren't provided, e.g. for OrderedPathParts (i.e. tuples) the third element is an identifier (as you can see in the country ones below, which is one of the few places that's actually used).

Which gets us to no_manip -- that says "don't just turn this into a string on normalization" -- the point here being that, when the last element of the normalized path isn't either a tuple (i.e. OrderedPathPart) or a PathPart instance, it'll automatically add an OrderedPathPart with default values to the end of the path, which prevents every single path inserting to an array (which is most of them, just not these) from needing a '(,)' stuck on the end.

I have a feeling there is a better way to do the bit with no_manip.