CollaboratorCheck

We need a Step to check whether the bot is a collaborator or not (for some operations it will need ``write`` access).

For instance, see how [CommanderCheck](https://github.com/amihaiemil/comdor/blob/master/src/main/java/co/comdor/github/CommanderCheck.java) is implemented (or [ArchitectCheck](https://github.com/amihaiemil/comdor/blob/master/src/main/java/co/comdor/github/ArchitectCheck.java)) -- it should be very similar. Don't forget about tests.