Disallow simultaneuous appearence of PR in several categories

[Semior001]

Make it optional, by flag. Default behavior: PR can be only in one category. Also, we have to specify the priority of matching PR to the category - either branch regexp or labels.

So, we will get the next set of possible options:

• Priority disabled, both regexp and labels may match a single PR to multiple categories;
• Priorities enabled, regexp first;
• Priorities enabled, labels first (priority among labels won't not considered);