Conditionally permit the console display of file covered >= threshold

Why ?

Two reasons :
* First time I used coco, I thought that the issue #35 was real
* For some people (me included), seing only the wrong is kind of depressing...
* It's easier to evaluate the overall coverage when what's greatly covered is displayed

So I added a small option to display in green files whose coverage is greater or equal to the threshold

I'll document the `threshold` argument…