fix #312, handle unaffiliated org role case from GitHub API (#313)

Author: tayloraswift

Sources/GitHubAPI/GitHub.OrganizationMembership.Role.swift

@@ -8,6 +8,9 @@ extension GitHub.OrganizationMembership
         case member = 0
         case admin = 1
         case billingManager = 2
+
+        /// This case exists in the wild but is not documented in the GitHub API reference.
+        case unaffiliated = 256
     }
 }
 extension GitHub.OrganizationMembership.Role:CustomStringConvertible
@@ -20,6 +23,7 @@ extension GitHub.OrganizationMembership.Role:CustomStringConvertible
         case .member:           "member"
         case .admin:            "admin"
         case .billingManager:   "billing_manager"
+        case .unaffiliated:     "unaffiliated"
         }
     }
 }
@@ -33,6 +37,7 @@ extension GitHub.OrganizationMembership.Role:LosslessStringConvertible
         case "member":          self = .member
         case "admin":           self = .admin
         case "billing_manager": self = .billingManager
+        case "unaffiliated":    self = .unaffiliated
         default:                return nil
         }
     }