Switch default from forced exit to no-exit

Allows `--exit` to override and force exit as before. Resolves mochajs#2879

bin/_mocha

@@ -94,7 +94,7 @@ program
   .option('--inspect-brk', 'activate devtools in chrome and break on the first line')
   .option('--interfaces', 'display available interfaces')
   .option('--no-deprecation', 'silence deprecation warnings')
-  .option('--no-exit', 'require a clean shutdown of the event loop: mocha will not call process.exit')
+  .option('--exit', 'force shutdown of the event loop after test run: mocha will call process.exit')
   .option('--no-timeouts', 'disables timeouts, given implicitly with --debug')
   .option('--no-warnings', 'silence all node process warnings')
   .option('--opts <path>', 'specify opts path', 'test/mocha.opts')