Use Istanbul as a CoverageCollector

This is a way I try to hook ``Istanbul``. I want to do something like you can switch to different collector and reporter, but I don't know what the reporter should be for the original ``cover``.
Can we make `text` the default?

I don't particularly like CLI's that write to disk unless explicitly asked to do that. If this is something you're particularly looking for, maybe you can add a config option for specifying a non-default output format.