support extras_require

Hey @reinout ,

I have products which have `extras_require` because they provide integration of other addons but do not require them. If the extras-packages are installed it will load additional ZCML (using `zcml:configure="installed other.package"`).

The dependencychecker lists those packages as missing requirements, which is a little annoying.

How could we handle this properly?

Maybe we could just kind of merge all extras_requires (excluding name "test.*") to the normal requirements for checking, so that they are no longer "missing".

I would spend some time on implementing this, but I'm not sure how it should work.
What do you think?

Cheers