Only generate HTML report on demand

[thekid]

Now that we have a summary display via #2, we could make generating the HTML report optional. Or maybe add an option to turn it off.

What do you think, @OlafSeng?

[OlafSeng]

Sure, I think it's a good idea to make the HTML report optional.

But I am still wondering what the default output should be. I would be prefer a file based summary, e.g. like this. And then maybe add an option to have only a summary for all files.

[thekid]

Here's an idea:

[OlafSeng]

Looks good!

[thekid]

So how about the following:

• We add the above output as default
• The current really short output can be used with --summary or something
• The HTML report is off by default, and can be activated with --report[=directory]

[thekid]

Released in https://github.com/xp-forge/coverage/releases/tag/v0.3.0