Sinatra CRUD

Routes to build

| HTTP Verbs | Routes | Actions | | GET | /posts | index | - DONE | GET | /posts/:id | show | - DONE | GET | /posts/new | new | - DONE | GET | /posts/:id/edit | edit | - DONE | POST | /posts | create | - DONE | DELETE | /posts/:id | delete | | PUT/PATCH | /posts/:id | update |