allow before hook failures to auto save screenshots

Before, it was impossible to get automatic screenshots to work when there was a failure in a `before` hook. This implements a custom mocha reporter that allows this logic to live at a higher level than the actual tests. In my local testing, it works as expected.

If we ever get [multiple reporters](https://github.com/visionmedia/mocha/pull/930), then we can allow arbitrary user-specified reporters again.

Addresses: https://github.com/groupon-testium/testium/issues/72