Add the "option" field type for custom fields

JIRA supports SelectList field types, which are similar to but
represented not quite the same as Strings. This change allows go-jira
to create issues with custom fields on them that are SelectLists.

issue.go

@@ -312,6 +312,12 @@ type TransitionPayload struct {
 	ID string `json:"id" structs:"id"`
 }
 
+// Option represents an option value in a SelectList or MultiSelect
+// custom issue field
+type Option struct {
+	Value string `json:"value" structs:"value"`
+}
+
 // UnmarshalJSON will transform the JIRA time into a time.Time
 // during the transformation of the JIRA JSON response
 func (t *Time) UnmarshalJSON(b []byte) error {
@@ -775,6 +781,10 @@ func InitIssueWithMetaAndFields(metaProject *MetaProject, metaIssuetype *MetaIss
 			issueFields.Unknowns[jiraKey] = IssueType{
 				Name: value,
 			}
+		case "option":
+			issueFields.Unknowns[jiraKey] = Option{
+				Value: value,
+			}
 		default:
 			return nil, fmt.Errorf("Unknown issue type encountered: %s for %s", valueType, key)
 		}