add mvc example

[XiXiangFiles]

add mvc example

[appleboy]

Why need this example?

[XiXiangFiles]

• Provide MVC example pros
  1. MVC is a popular pattern for web dev.
  2. other languages provide it. such as nodejs: express, golang: iris... so if dev user first time use gin and want to use MVC pattern, they can directly use this example, we can make developers familiar with gin faster if we provide it.

So I want to contribute this example on this repo for all user. @appleboy