feature: enable plugins / custom middleware

[anoack93]

As a developer I want to customize api responses or add project specific logic by adding a plugin.

Suggestion:

First enable a configuration file .servemocks.config.js which will be loaded by servemocks,
This could look like this:

import customMiddleware from './foo/bar/custom-middleware'

export default {
  middleware: [
    {
      name: 'custom-middleware',
      handler: customMiddleware
    }
  ]
}

Servemocks now will be able to execute customMiddleware at some point.