fix(cli): handle extends setting properly

Author: manuth

lib/cli/config.js

@@ -11,6 +11,7 @@ const fs = require('fs');
 const path = require('path');
 const debug = require('debug')('mocha:cli:config');
 const findUp = require('find-up');
+const yargs = require('yargs/yargs');
 const {createUnparsableFileError} = require('../errors');
 const utils = require('../utils');
 
@@ -76,6 +77,10 @@ exports.loadConfig = filepath => {
     } else {
       config = parsers.json(filepath);
     }
+
+    config = yargs(undefined, path.dirname(filepath))
+      .parserConfiguration(require('./options').YARGS_PARSER_CONFIG)
+      .config(config);
   } catch (err) {
     throw createUnparsableFileError(
       `Unable to read/parse ${filepath}: ${err}`,