Ability to update posts

[Savjee]

Right now, I can update posts, but that isn't reflected anywhere.

Solution:

• Upgrade to Elevent 2.0 once it's final
• Use git to figure out when a post was written, and last updated
  • Ref: https://github.com/11ty/eleventy/issues/869

Todo:

• [x] Upgrade to Eleventy 2.0
• [ ] Show "Last modified" date at the bottom of each post (if it's not the same as publication date)
• [ ] Update the rich snippet accordingly, so Google knows the post was updated.