Issue 596/include only assigned (#817)

* Add new 'include-only-assigned' option

If set, only issues containing assignees will be processed

* Test new flag

* Update code comment

* Update src/classes/issues-processor.ts

Co-authored-by: Francesco Renzi <rentziass@github.com>

* Update index.js with typo fix

Co-authored-by: Francesco Renzi <rentziass@github.com>

src/interfaces/issues-processor-options.ts

@@ -52,4 +52,5 @@ export interface IIssuesProcessorOptions {
   ignorePrUpdates: boolean | undefined;
   exemptDraftPr: boolean;
   closeIssueReason: string;
+  includeOnlyAssigned: boolean;
 }