Add close-issue-reason option (#764)

* Add close-as-not-planned option

* update readme

* add to Option enum

* improve wording

* npm run pack

* updates from review

* fix tests and improve error message

* fix readme order

src/interfaces/issues-processor-options.ts

@@ -51,4 +51,5 @@ export interface IIssuesProcessorOptions {
   ignoreIssueUpdates: boolean | undefined;
   ignorePrUpdates: boolean | undefined;
   exemptDraftPr: boolean;
+  closeIssueReason: string;
 }