Add "contents" interface to the componant

This PR adds a contents interface that allows you to bind to the actual value of the input.

This is useful for the case where you want access to the text context inside of the type ahead. My use case is to create a type ahead that allows users to select an existing entity, or create a new one.

I tested this both in the application I am currently working on, and it works without regressions.

I also updated the readme to document this feature.

I did not build the dist, or bump the version as I figured I'd let the maintainer do the honors, but this does feel like a minor version bump to me.

While I was at it, I also cached the jQuery input selection for performance.