allow global configuration with ~/.simplecov

I posted a [message to the mailing list](https://groups.google.com/d/topic/simplecov/Y2oZRGwyXRg/discussion) about this, then realized it was a simple change to implement. Essentially the change loads `~/.simplecov` prior to a local `.simplecov` allowing global configurations to be set.

I took at stab at writing a test but am having difficulty with the appraisal gem (I believe for the same reason as mentioned in the mailing list post). Looking forward to feedback.