Initializing Docsearch more than once creates multiple overlapping autocomplete drop downs

**Do you want to request a _feature_ or report a _bug_?**
Feature

**What is the current behavior?**
When calling `window.docsearch({...options})` multiple times, multiple autocomplete dropdowns are added to the UI.

**If the current behavior is a bug, please provide all the steps to reproduce and a minimal
[JSFiddle][3] example or a repository on GitHub that we can `npm install`
and `npm start`.**

**What is the expected behavior?**
I need to be able to apply filters to the search based on user selection. I'm not sure if this is currently possible. I have tried to call `window.docsearch({...options})` when the user picks a category to search by, but that seems to add multiple autocomplete dropdowns instead of refreshing the current instance. It would be nice if you could refresh the autocomplete dropdown with results from a newly filtered index.

Start at the public window.docsearch({...options}) entry point and reproduce the problem by invoking it repeatedly after a category selection. Trace how the autocomplete dropdown and filtered index are managed; done means reusing or refreshing the current instance so one dropdown reflects the selected filter.

Written by the indexing model from the issue text.