Allow *default-reporter* to be an actual reporter object.

[tmccombs]

Currently, the default-reporter variable must be a keyword, which is converted into a symbol naming a class, and an instance of that class is created every time a reporting function is called without an explicit reporter.

This has a couple negative implications:

1. The default reporter can't be an object created with constructor arguments.
2. The default reporter can't maintain state between calls to reporting functions

I propose that default-reporter should also work if you store an instance of the reporter class. See https://github.com/tmccombs/prove/tree/object-reporters.