Implement SQL commenter as a Sequelize wrapper (#2)

* implement sequelize plugin as wrapper

* fix test files; address MR comments

.gitignore

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+# Dependency directories
+node_modules/
+.idea

README.md

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+# sqlcommenter-sequelize
+
+sqlcommenter is a plugin/middleware/wrapper to augment SQL statements from Sequelize.js
+with comments that can be used later to correlate user code with SQL statements.
+
+It supports Node v6 and above to use ES6 features.
+
+This version of the plugin is a minimal specific implementation of the google-cloud 
+sqlcommenter plugin. See the original documentation here: https://google.github.io/sqlcommenter/node/sequelize/
+
+### Installation
+```shell
+yarn add wanderlog/sqlcommenter-sequelize
+```
+
+### Usage
+```javascript
+const Sequelize = requrie('sequelize');
+const {wrapSequelize} = require('wanderlog/sqlcommenter-sequelize');
+
+const db = new Sequelize({...});
+wrapSequelize(db);
+```

index.js

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+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This is a pared down version of the Google sqlcommenter-sequelize plugin,
+// meant to comment just some relevant stacktrace.
+//
+// The original documentation can be found here:
+// https://google.github.io/sqlcommenter/node/sequelize/
+//
+// The original project can be found here:
+// https://github.com/google/sqlcommenter/tree/master/nodejs/sqlcommenter-nodejs/packages/sqlcommenter-sequelize
+
+const {hasComment, makeMinimalUsefulStacktrace} = require('./util');
+
+/**
+ * Adds a comment about the stacktrace. Helps to figure out where a
+ * SQL query originates.
+ *
+ * @param {Object} sequelize
+ * @return {void}
+ */
+exports.wrapSequelize = (sequelize) => {
+
+    /* c8 ignore next 2 */
+    if (sequelize.___alreadySQLCommenterWrapped___)
+        return;
+
+    const run = sequelize.dialect.Query.prototype.run;
+
+    // Please don't change this prototype from an explicit function
+    // to use arrow functions lest we'll get bugs with not resolving "this".
+    sequelize.dialect.Query.prototype.run = function(sql, sql_options) {
+
+        // If a comment already exists, do not insert a new one.
+        // See internal issue #20.
+        if (hasComment(sql)) // Just proceed with the next function ASAP
+            return run.apply(this, [sql, sql_options]);
+
+        // Allow only alphanumeric, periods, slashes, dashes, underscores,
+        // spaces, newlines. The main concern is preventing injection of '*/
+        // within the stacktrace.
+        const commentStr = `stacktrace='${makeMinimalUsefulStacktrace()}'`.replace(/[^\w.:/\\\-\s\n]/g, '');
+
+        if (commentStr && commentStr.length > 0)
+            sql = `${sql} /*${commentStr}*/`;
+
+        return run.apply(this, [sql, sql_options]);
+    };
+
+    // Finally mark the object as having already been wrapped.
+    sequelize.___alreadySQLCommenterWrapped___ = true;
+}