Add support for CWeek DateTime format

[spariev]

Fix for #422

I've added a case clause for cwyear and cweek as per example in #415
Also added a corresponding test

[joshuacronemeyer]

@spariev this does not handle the scenario where the year is not part of the date string

3.0.1 :006 > Date._parse('W53-6 13:00:01')
 => {:hour=>13, :min=>0, :sec=>1, :cweek=>53, :cwday=>6}

[joshuacronemeyer]

Haha. Also I forgot to say thank you!!! It's great you're looking at this.

[joshuacronemeyer]

Oh and one other note we've just had a bug report about the way we handle these kinds of date strings. The way we're just passing in the mocked year into Date.new isn't correct. During the last week of the year the year might need to be the NEXT year. I'm not 100% sure how we're going to fix it but I think it's going to be getting the year from the un-mocked method, computing the year offset of the mocked date and then adding it to the result of the un-mocked method.

#434

[spariev]

@joshuacronemeyer no problem, thanks for the direct feedback 😂
I'll try fix the case with cweek later this week (doh), your approach seems reasonable enough, maybe we will fix the related bug along the way

[spariev]

@joshuacronemeyer I see that in #437 there is a fix for cweek as well, so I'll wait until you guys are done with it and then we'll see what to do with this PR.