feat(assignees): add 6 new options to avoid stale for assignees (#327)

* feat(assignees): add new option to avoid stale for assignees

closes #271

* test: add more coverage

* docs: fix readme format issue

* docs: reorder and enhance typo

* docs(contributing): add more information about the npm scripts

src/interfaces/issue.ts

@@ -1,4 +1,5 @@
 import {IsoDateString} from '../types/iso-date-string';
+import {IAssignee} from './assignee';
 import {ILabel} from './label';
 import {IMilestone} from './milestone';
 
@@ -12,4 +13,5 @@ export interface IIssue {
   state: string;
   locked: boolean;
   milestone: IMilestone | undefined;
+  assignees: IAssignee[];
 }