feat(typeahead): min-length value of 0, and new option open-on-click

* Properly handles min-length value of 0 (keeping typeahead popup open even when input is empty string)
* Adds a new open-on-click option, which defaults to false.  When true, Will go ahead and open typeahead popup (or start timeout to do so) when user clicks in input and does not start typing.  This is useful for when typeahead is being used not just to autocomplete, but as a way of giving the user a hint of common/desired/allowed values.

This PR contains code, tests, and documentation, and works as-is on the bootstrap3 branch.