[config] clean up logic in config locator

- exported simple field can't be changed (and it's not a good idea to
  export object)
- less return and no process.exit in non entry libraries

example/.gitignore

@@ -0,0 +1 @@
+build

lib/cli.js

@@ -33,6 +33,11 @@ module.exports = {
         logger.debug('cli input', cli.input);
         logger.debug('cli flags', cli.flags);
 
-        configLocator.detectConfigFile(cli.flags);
+        let configFilePath = configLocator.detectConfigFile(cli.flags);
+        if (configFilePath === '') {
+            process.exit(1);
+        }
+
+        logger.debug('keep moving!');
     }
 };

lib/config/locator.js

@@ -3,17 +3,26 @@
  */
 const logger = require('../logger');
 const fsUtil = require('../util/fs');
+const DEFAULT_CONFIG_FILE = '.about.yml';
 
 function detectConfigFile(flags) {
     if (typeof flags.config === 'string') {
         let configFile = flags.config;
         logger.debug(`config file set as ${configFile}`);
-        // it must exist, if not we fallback to the default one and throw a warning
-        if (!fsUtil.fileExists(configFile)) {
-            logger.warn(`config file can't be found ${configFile}`);
+        if (fsUtil.fileExists(configFile)) {
+            return configFile;
         }
+        logger.warn(`specified config file can't be found`, {file: configFile});
     }
-    logger.debug('using default config file');
+    logger.debug('try to use default config file');
+    // TODO: may need to add cwd
+    if (fsUtil.fileExists(DEFAULT_CONFIG_FILE)) {
+        return DEFAULT_CONFIG_FILE;
+    } else {
+        logger.warn(`default config file can't be found`, {file: DEFAULT_CONFIG_FILE});
+    }
+    logger.error('no config file found');
+    return '';
 }
 
 module.exports = {