Style <hr> elements to match borders on other elements

I've noticed that Minima doesn't style the `<hr>` element by default, so horizontal rules look obtrusive when compared to existing borders, like the one at the bottom of post headers.

I'd like to see something like this instead:
```
hr {
    border: 0;
    border-bottom: 1px solid $border-color-01;
}
```

I'm happy to open a PR if this can be merged.