Add ability for Typeahead to take an async-function

[benirose]

I am using my own api client, but would like to use the typeahead to pull async results. Will you accept a PR with an added async-function property, which is a function that returns a promise, which resolves with data (as the getJSON method currently used for URL async does). I wanted to ask before I put in the work.

[benirose]

Just kidding, I already did the work in a local copy of the component, so I may as well submit the PR 😆