Option to force prefixes to be included when completing

This is related to #40/#44, but instead of just in terms of what's shown, this is about what's completed. I use autocomplete-modules (which partially duplicates this), but it'd be much easier to use in browser-only ES6 projects if I could force the extension to show, regardless of what the scope says.

Start by tracing how completion entries are built and how scope controls displayed prefixes, then review related issues #40 and #44 and compare autocomplete-modules. Done means an option can force prefixes to appear in completions for browser-only ES6 projects regardless of scope.

Written by the indexing model from the issue text.