support multiple report formats in one test run

Hi,

Firstly my use case.  I always want the html diff report, however I want to run these tests from a framework and having the json report is also useful for machine parsing and exiting with the correct exit code.

Therefore I would like to have the option of both reports.

Commander doesn't seem to really support this notion, they do have an example using a <list> see here:

https://github.com/visionmedia/commander.js/blob/master/examples/defaults

However this feels a little clumsy as relies on the user passing args in the correct format e.g. ```-o html,json``` without spaces etc.

This approach accesses the rawArgs property...maybe not great but see what you think. I have ensured tests still pass.

Thanks,

Matt.