ManageIncidents - support set resolution

[hagaishapira]

In case of incident resolution (status is resolved) the API allows to set a resolution string.
As far as I can see, this is currently not supported.

[theckman]

@hagaishapira would you be willing / able to get a PR raised to add that field to this type:

go-pagerduty/incident.go

Lines 155 to 162 in 69ade4b

	// 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"`
	}

The reason I ask is that I have permission to review and merge the PRs of others, and not my own, so an external contribution is easier for me to shepherd towards release.

[theckman]

@hagaishapira I went ahead and raised a PR since I didn't hear back from you. Your request should make it as part of v1.4.0 when it's eventually released.