Add option to disable built-in JS

[lulis]

Added option to disable the js/simplesearch.js built-in javascript. Just like we can do with built-in CSS. Useful when someone wanna use custom JS or another search twig templates. Well, at least, I need it. So there is the patch ;)

[lulis]

Done, @rhukster . But I'm not using admin interface, so I cannot check if this blueprint is fully functional. Hope it works.

[rhukster]

looks good!