Support for lower-level queries on indexedDB

superfeedr has done a nice job with this backbone -> indexedDB adapter,
but their query interface is somewhat limited. This commit adds an
alternate interface that lets us specify the index and cursor bounds we
want. This interface requires deeper knowledge of indexedDB indices, but
is more powerful overall.

components/indexeddb-backbonejs-adapter/backbone-indexeddb.js

@@ -419,6 +419,25 @@
                         }
                     }
                 });
+            } else if (options.index) {
+                index = store.index(options.index.name);
+                if (index) {
+                    if (options.index.lower && options.index.upper) {
+                        bounds = IDBKeyRange.bound(options.index.lower, options.index.upper);
+                    } else if (options.index.lower) {
+                        bounds = IDBKeyRange.lowerBound(options.index.lower);
+                    } else if (options.index.upper) {
+                        bounds = IDBKeyRange.upperBound(options.index.upper);
+                    } else if (options.index.only) {
+                        bounds = IDBKeyRange.only(options.index.only);
+                    }
+
+                    if (typeof options.index.order === 'string' && options.index.order.toLowerCase() === 'desc') {
+                        readCursor = index.openCursor(bounds, window.IDBCursor.PREV || "prev");
+                    } else {
+                        readCursor = index.openCursor(bounds, window.IDBCursor.NEXT || "next");
+                    }
+                }
             } else {
                 // No conditions, use the index
                 if (options.range) {

js/components.js

@@ -21777,6 +21777,25 @@ return jQuery;
                         }
                     }
                 });
+            } else if (options.index) {
+                index = store.index(options.index.name);
+                if (index) {
+                    if (options.index.lower && options.index.upper) {
+                        bounds = IDBKeyRange.bound(options.index.lower, options.index.upper);
+                    } else if (options.index.lower) {
+                        bounds = IDBKeyRange.lowerBound(options.index.lower);
+                    } else if (options.index.upper) {
+                        bounds = IDBKeyRange.upperBound(options.index.upper);
+                    } else if (options.index.only) {
+                        bounds = IDBKeyRange.only(options.index.only);
+                    }
+
+                    if (typeof options.index.order === 'string' && options.index.order.toLowerCase() === 'desc') {
+                        readCursor = index.openCursor(bounds, window.IDBCursor.PREV || "prev");
+                    } else {
+                        readCursor = index.openCursor(bounds, window.IDBCursor.NEXT || "next");
+                    }
+                }
             } else {
                 // No conditions, use the index
                 if (options.range) {