Merge tag reference abstraction/partial

Perhaps helpers.js would be a more suitable place for
getDefaultMergeTags() and getListMergeTags() … ?

lib/tools.js

@@ -20,6 +20,8 @@ module.exports = {
     formatMessage,
     getMessageLinks,
     prepareHtml,
+    getDefaultMergeTags,
+    getListMergeTags,
     workers: new Set()
 };
 
@@ -222,3 +224,73 @@ function prepareHtml(html, callback) {
         return callback(null, juice(preparedHtml));
     });
 }
+
+function getDefaultMergeTags(callback) {
+    // Using a callback for the sake of future-proofness
+    callback(null, [
+        {
+            key: 'LINK_UNSUBSCRIBE',
+            value: 'URL that points to the unsubscribe page'
+        }, {
+            key: 'LINK_PREFERENCES',
+            value: 'URL that points to the preferences page of the subscriber'
+        }, {
+            key: 'LINK_BROWSER',
+            value: 'URL to preview the message in a browser'
+        }, {
+            key: 'EMAIL',
+            value: 'Email address'
+        }, {
+            key: 'FIRST_NAME',
+            value: 'First name'
+        }, {
+            key: 'LAST_NAME',
+            value: 'Last name'
+        }, {
+            key: 'FULL_NAME',
+            value: 'Full name (first and last name combined)'
+        }, {
+            key: 'SUBSCRIPTION_ID',
+            value: 'Unique ID that identifies the recipient'
+        }, {
+            key: 'LIST_ID',
+            value: 'Unique ID that identifies the list used for this campaign'
+        }, {
+            key: 'CAMPAIGN_ID',
+            value: 'Unique ID that identifies current campaign'
+        }
+    ]);
+}
+
+function getListMergeTags(listId, callback) {
+    let lists = require('./models/lists');
+    let fields = require('./models/fields');
+
+    lists.get(listId, (err, list) => {
+        if (err) {
+            return callback(err);
+        }
+        if (!list) {
+            list = {
+                id: listId
+            };
+        }
+
+        fields.list(list.id, (err, fieldList) => {
+            if (err && !fieldList) {
+                fieldList = [];
+            }
+
+            let mergeTags = [];
+
+            fieldList.forEach(field => {
+                mergeTags.push({
+                    key: field.key,
+                    value: field.name
+                });
+            });
+
+            return callback(null, mergeTags);
+        });
+    });
+}