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" />``` 
I think it's safer and more consistent to do same check as above (on line 60, to check whether the attrs expression is truthy/not empty) as that would work with any falsey value.