Provide way to not clear screen

[mvz]

I'm actually trying to resolve #93, and a lot of interesting output from the build scrolls by but is then cleared completely, leaving only the output from one test case. An option to not clear the screen during mutest run would be helpful here.

[dgollahon]

I think there is another report option for use in CI contexts that doesn't overwrite the screen. I don't recall the flag off the top of my head though.

[mvz]

I investigated a bit and you can set CI in the environment.