feat: config can be a function

lib/getConfig.js

@@ -18,7 +18,10 @@ const findOverrides = (root) => {
     const filename = path.resolve(dir, file);
 
     if (fs.existsSync(filename) && fs.statSync(filename).isFile()) {
-      return require(filename);
+      return {
+        dir,
+        overrides: require(filename)
+      };
     }
   }
 
@@ -31,7 +34,10 @@ const findOverrides = (root) => {
       const changelog = require(pkgFilename).config.commitizen.changelog;
 
       if (changelog) {
-        return changelog;
+        return {
+          dir,
+          overrides: changelog
+        };
       }
     // eslint-disable-next-line no-empty
     } catch (error) {}
@@ -41,11 +47,45 @@ const findOverrides = (root) => {
     return findOverrides(parent);
   }
 
-  return {};
+  return {
+    dir,
+    overrides: null
+  };
+};
+
+const getConfigContext = (currentConfigDir) => {
+  const memo = {parentConfig: null};
+
+  return {
+    defaultConfig: defaults,
+    get parentConfig () {
+      if (memo.parentConfig) {
+        return memo.parentConfig;
+      }
+
+      const parent = path.resolve(currentConfigDir, '..');
+
+      // eslint-disable-next-line no-use-before-define
+      memo.parentConfig = getConfig(parent);
+
+      return memo.parentConfig;
+    }
+  };
+};
+
+const getOverrides = (root) => {
+  // eslint-disable-next-line prefer-const
+  let {overrides, dir: overridesDir} = findOverrides(root);
+
+  if (typeof overrides === 'function') {
+    overrides = overrides(getConfigContext(overridesDir));
+  }
+
+  return overrides;
 };
 
 const getConfig = (root) => {
-  const overrides = findOverrides(root);
+  const overrides = getOverrides(root);
 
   if (typeof overrides !== 'object') {
     signale.fatal(new TypeError('Expected changelog config to be an object.'));