Support relative path as a reporter name

In the Mocha code there is the following bit of logic: https://github.com/mochajs/mocha/blob/master/lib/mocha.js#L234

it allows customers to write local reporters and provide them to mocha. This feature is even used in this package in the [verify script](https://github.com/glenjamin/mocha-multi/blob/master/verify).

However it is not working in `mocha-multi`

```
mocha -R mocha-multi --reporter-options spec=-,./my-custom.js=out.txt
```