Swap tape for prova

Prova uses tape, but also makes it better to test in browser
if needed, and gives you better errors.
NPM fetch fallback, while still loading cache

So we query npm registry directly with a streaming interface,
which allows us to add packages to the ui as they come.

Also added a loader to the ui, to let the user know that
something is happening.

TODO: make loadCache streaming for cache update from npm.