Add ability to set Escalation Policy when managing incidents

This was originally suggested by @evnsio in #273, but there were some challenges
getting that PR shephered through and so this is being reintroduced as a new PR.

Replaces #273

incident.go

@@ -166,12 +166,13 @@ type CreateIncidentOptions struct {
 
 // ManageIncidentsOptions is the structure used when PUTing updates to incidents to the ManageIncidents func
 type ManageIncidentsOptions struct {
-	ID          string        `json:"id"`
-	Type        string        `json:"type"`
-	Status      string        `json:"status,omitempty"`
-	Priority    *APIReference `json:"priority,omitempty"`
-	Assignments []Assignee    `json:"assignments,omitempty"`
-	Resolution  string        `json:"resolution,omitempty"`
+	ID               string        `json:"id"`
+	Type             string        `json:"type"`
+	Status           string        `json:"status,omitempty"`
+	Priority         *APIReference `json:"priority,omitempty"`
+	Assignments      []Assignee    `json:"assignments,omitempty"`
+	EscalationPolicy *APIReference `json:"escalation_policy,omitempty"`
+	Resolution       string        `json:"resolution,omitempty"`
 }
 
 // MergeIncidentsOptions is the structure used when merging incidents with MergeIncidents func