New feature: pretty relative urls

[mvolfik]

Hey Frozen-Flask team! I was going through my old issues and decided to have another shot at a feature request I made here 4 years ago. I'm too deep into JS now, so I actually don't know if kids new to programming these days still build websites with Python and then host them on static servers like I used to, but if they do, now they can have pretty URLs without the ugly /index.html just like I wanted to.

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Why it's necessary to first append the /index.html and then remove it (i.e. why the approach I said I tried in the original issue didn't work): if the index.html isn't there, relpath resolves a path to the directory itself, which obviously doesn't include the trailing slash. But after that it isn't anymore possible to disambiguate if the resolved URL is a directory ../example, or a file ../example. So we need to keep it, resolve the relpath, and then again strip query+fragment, check for index.html, remove it and then put everything back together again.

Fixes #90