feat(statistics): display some stats in the logs (#337)

* test: add more coverage

* docs: reorder and enhance typo

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

* feat(statistics): add simple statistics

* feat(statistics): add more stats

* refactor(issues-processor): remove some options from the constructor

it should have been only useful for the tests

* feat(statistics): add stats for new stale or undo stale issues

* chore(rebase): handle rebase conflicts

__tests__/classes/issues-processor-mock.ts

@@ -0,0 +1,38 @@
+import {Issue} from '../../src/classes/issue';
+import {IssuesProcessor} from '../../src/classes/issues-processor';
+import {IComment} from '../../src/interfaces/comment';
+import {IIssuesProcessorOptions} from '../../src/interfaces/issues-processor-options';
+
+export class IssuesProcessorMock extends IssuesProcessor {
+  constructor(
+    options: IIssuesProcessorOptions,
+    getActor?: () => Promise<string>,
+    getIssues?: (page: number) => Promise<Issue[]>,
+    listIssueComments?: (
+      issueNumber: number,
+      sinceDate: string
+    ) => Promise<IComment[]>,
+    getLabelCreationDate?: (
+      issue: Issue,
+      label: string
+    ) => Promise<string | undefined>
+  ) {
+    super(options);
+
+    if (getActor) {
+      this.getActor = getActor;
+    }
+
+    if (getIssues) {
+      this.getIssues = getIssues;
+    }
+
+    if (listIssueComments) {
+      this.listIssueComments = listIssueComments;
+    }
+
+    if (getLabelCreationDate) {
+      this.getLabelCreationDate = getLabelCreationDate;
+    }
+  }
+}