Add 404 page to default theme

[joerter]

Fixes #37

Turns out I didn't have to make any changes to the render! function since 404.html doesn't match #"/$". I can make that more explicit though if needed.

I also ripped the format straight from biffweb.com's 404 page, if that looks familiar at all haha.

[jacobobryant]

Ah, you are right! I read the render! function a little too hastily. Thanks for the PR.