support mocha require multi modules in cli

support mocha cli command like this:

```
mocha -r should -r blanket --report travis-cov test/*.test.js
```
Fixing fs.existsSync issues in older versions of node
Merge pull request #375 from jonathanKingston/master

Fixing fs.existsSync issues in older versions of node
Merge pull request #371 from dead-horse/support-multi-mocha-require

support mocha require multi modules in cli