Add `include` to multispec matrix

[frenzymadness]

This adds a possibility to use include in a multispec matrix which is useful when you have a lot of possible combinations but you are interested only in a few of them. In that case, include is also easier to maintain when you add a new version into the multispec.

I did a little bit of refactoring because the method check_matrix_combinations had multiple purposes – it returned True/False for all matrix combinations and a data structure for combination_extras.

And now:

• Function is_in_matrix returns True/False if a combination of the distro and all the selectors is found in matrix. It does not care about include/exclude, that's handled on verify_selectors level.
• Function get_extras is there only for combination_extras returning all structures for which distro and selectors combinations fit.

Having both include and exclude in a single matrix is prohibited.

Tests are adjusted and some of them are new for the new functionality.

Fixes: #128

[frenzymadness]

Thank you for this PR. I did not find any issue with it. LGTM.

Thanks for the review! Will we wait for another one from the team?

[praiskup]

Will we wait for another one from the team?

No need to, my 2c. But +1 from me, too.

[zmiklank]

Great, I will also create a new release today or tomorrow.