javascript_include_tag "extname" option for use by asset_path

javascript_include_tag uses ActionView::Helpers.asset_path to compute the path for javascript files.  However asset_path automatically appends an extension onto the path which is controlled by the extname option. 

javascript_tag_helper only passed the 'protocol' option to asset_path, making it impossible to modify the extname behavior.  Without this, underscore JST template files are getting a '.js' extension appended to them.

This breaks the 'jammit' gem since it calls javascript_include_tag internally.   

For instance ```javascript_include_tag('core.jst')``` is returning ```<script src="/javascripts/core.jst.js"></script>```