Set `border-style` for `<hr>` elements

[Richienb]

By default, a <hr> element is styled with border-style: inset; which makes it turn out like this:

By adding border-style: solid; we can make the line act like users would expect it to.

[sindresorhus]

Do different browsers render it differently? Or do you just want a better look for all browsers?

[Richienb]

Or do you just want a better look for all browsers?

Might be good for #13

[sindresorhus]

Yes