Add support for installing a set of phrases from URI

This is something I've been wanting to do for a while. Quikey needs support for installing prebuilt phrases from a URI (file, web, etc). I haven't thought out what format the files would be in or what they would be called (phrasebook, package, bundle, etc).

This issue is the beginning of a public document to track it.

The issue names no files, tests, or entry points. Start by clarifying the supported URI sources and the format and name of phrase sets; done means the installation behavior and package format have an agreed design before implementation begins.

Written by the indexing model from the issue text.