Pass arguments to the reporter constructor

This allows to pass arguments to the test reporter constructor from the configuration object.

In my use case I'm trying to use `testem` as a normal node module.
I can `require('testem')` and set most of the configuration options I need.

However it's impossible to specify the arguments of the test reporter constructor.

Main reason why I need this is because I'm trying to drive testem from a script and I can't `spawn` processes because stdout is not drained correctly in node 0.10 and testem never exits.