Add options to specify mandatory reviewer in path

[Wingless-Archangel]

This will add the functionality to add mandatory users and/or groups to enforce review before able to merge. Please find the part of sample configuration below.

wz-branch-reviewers:
  - branch-id: master
    users: []
    groups: []
    mandatory-users: [user1]
    mandatory-groups: [group1]
    paths:
      - path: path1/*
        users:
          - user2
          - user1
        groups: []
        mandatory-users: []
        mandatory-groups: [group1]
      - path: path2/*
        users:
          - user3
        groups: []
        mandatory-users: [user4, user1]
        mandatory-groups: []

[CLAassistant]

All committers have signed the CLA.

[sireliah]

We additionally tested the change using a real repository and everything seems to be in order. However it would be great to run proper tests as well.

[robertoaloi]

Property-based tests correctly pass, thanks!