Add FruitDOWN as a supported adapter

[nolanlawson]

I wrote this thing: https://github.com/nolanlawson/fruitdown

Presumably this is much more useful to our users than idb-alt was, so now that we've gotten rid of it, we can replace it with pouchdb.fruitdown.js and offer a very useful adapter that actually works in environments like WKWebView and allows unlimited storage in iOS/Safari.

I might not have time to do this today, but anybody who wants to add this thing only needs to look at 427efbc (which removed idb-alt) and basically revert it, but use fruitdown instead of level-js. Also for the adapter name, I'd suggest just calling it 'fruitdown'.

[nolanlawson]

fixed by 25da9e5 and a0fc6a9