feat(config): add 'extends' configuration option

Give users the option to extend Options with a deep merge with the object inside the "extends"
configuration option

fix commitizen#145

index.js

@@ -3,6 +3,7 @@
 var engine = require('./engine');
 var conventionalCommitTypes = require('conventional-commit-types');
 var configLoader = require('commitizen').configLoader;
+var merge = require('lodash.merge');
 
 var config = configLoader.load() || {};
 var options = {
@@ -28,6 +29,10 @@ var options = {
     100
 };
 
+if (!!config.extends) {
+  options = merge(options, config.extends);
+}
+
 (function(options) {
   try {
     var commitlintLoad = require('@commitlint/load');

package.json

@@ -25,6 +25,7 @@
     "commitizen": "^4.0.3",
     "conventional-commit-types": "^3.0.0",
     "lodash.map": "^4.5.1",
+    "lodash.merge": "^4.6.2",
     "longest": "^2.0.1",
     "word-wrap": "^1.0.3"
   },