store SingleFileCore in this

lib/single-file/single-file-core.js

@@ -18,7 +18,7 @@
  *   along with SingleFile.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-const SingleFileCore = (() => {
+this.SingleFileCore = (() => {
 
 	const SELECTED_CONTENT_ATTRIBUTE_NAME = "data-single-file-selected-content";
 	const REMOVED_CONTENT_ATTRIBUTE_NAME = "data-single-file-removed-content";