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

__tests__/constants/default-processor-options.ts

@@ -50,5 +50,6 @@ export const DefaultProcessorOptions: IIssuesProcessorOptions = Object.freeze({
   ignoreUpdates: false,
   ignoreIssueUpdates: undefined,
   ignorePrUpdates: undefined,
-  exemptDraftPr: false
+  exemptDraftPr: false,
+  closeIssueReason: ''
 });