title: Add percent to ngModel value Angular 6 (pipe)
body: <p>I would like to have value in input (from ngModel) and in the end have percent symbol '%'. When I try do it using custom pipe, on the begining everything is fine, but when I remove value from input and change it, this percent is added every time you enter the keyboard. How can I change it? I need allow type eg '40' and after this add '%', no immediately after '4'.
  Example: 
  <a href="https://stackblitz.com/edit/angular-ppkbwn?file=src%2Fapp%2Fapp.module.ts" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-ppkbwn?file=src%2Fapp%2Fapp.module.ts</a></p>