Bug fixes and enhancements

This pull request addresses bugs and missing features that were encountered when trying to use your plugin with the  searchbox.io service:

 * Fixes the autocomplete query where the indexName wasn't passed into the query, causing an error
 * Makes autocomplete an option within the plugin so js code isn't automatically added to the site for users who don't want that feature or want to enable their own implementation
 * Uses the plugin name with plugins_url() instead of __FILE__ so that a symlinked plugin directory does not break functionality and cause a 404 error to the js file
 * Searches the post_content field as well as the title when using the autocomplete search
 * Adds support for processing shortcodes when indexing data, otherwise shortcodes are stored in elasticsearch and can break functionality for many sites (like sites with recipes)