_announcements: Add week and date to metadata

_announcements/week-0.md

 ---
 title: Week 0 Announcement
+week: 0
+date: 2019-04-01
 ---
 
 Hello world!

_announcements/week-1.md

 ---
 title: Week 1 Announcement
+week: 1
+date: 2019-04-08
 ---
 
 1. Create a [new repository based on Just the Class](https://github.com/kevinlin1/just-the-class/generate).