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/classes/issue.spec.ts

@@ -62,7 +62,8 @@ describe('Issue', (): void => {
       ignoreIssueUpdates: undefined,
       ignorePrUpdates: undefined,
       exemptDraftPr: false,
-      closeIssueReason: ''
+      closeIssueReason: '',
+      includeOnlyAssigned: false
     };
     issueInterface = {
       title: 'dummy-title',