Add article list viewing feature

Visitors can now view a list of articles sorted by creation time (descending), in addition to creating new articles.

As with the previous feature, I'll make work on the visuals/aesthetics once I have this feature branch merged with master.

Resolves #3 .
If you're doing this at the start of all these `it` blocks, you can pull it into a `before` block below your `let` statements:
```
before { visit '/articles' }
```