typeahead-focus-first=false option to prevent first match from being focused

Fixes #908.

Should be 100% backwards compatible with existing markup and functionality.

Current functionality (first result is automatically focused as you type):
``` <input typeahead="..."  /> ``` 

New functionality (first result is NOT automatically focused as you type):
``` <input typeahead="..."  typeahead-focus-first="false" />``` 
Missing a test for when pressing enter or tab when `activeIdx == -1`, nothing should happen and nothing should be selected.