Middleware to configure response headers

[phartenfeller]

Hi, first of thanks for this project I use it a lot :) !

This PR adds a middleware that lets you specify response HTTP Headers for certain paths and filetypes. With a JSON config that gets mounted into the container multiple header rules can be specified.

I added a documentation file for more info on how to use it.

Example config for implementing the caching best practices from gatsby.js :

{
  "configs": [
    {
      "path": "/page-data",
      "fileExtension": "json",
      "headers": [
        {
          "key": "cache-control",
          "value": "public, max-age=0, must-revalidate"
        }
      ]
    },
    {
      "path": "*",
      "fileExtension": "html",
      "headers": [
        {
          "key": "cache-control",
          "value": "public, max-age=0, must-revalidate"
        }
      ]
    },
    {
      "path": "/static/",
      "fileExtension": "*",
      "headers": [
        {
          "key": "cache-control",
          "value": "public, max-age=31536000, immutable"
        }
      ]
    },
    {
      "path": "*",
      "fileExtension": "js",
      "headers": [
        {
          "key": "cache-control",
          "value": "public, max-age=31536000, immutable"
        }
      ]
    },
    {
      "path": "*",
      "fileExtension": "css",
      "headers": [
        {
          "key": "cache-control",
          "value": "public, max-age=31536000, immutable"
        }
      ]
    }
  ]
}

Maybe take a closer look to the code as this is my first time programming in Go :) .

Would solve #24

[PierreZ]

Thanks a lot for the PR, I just added some changes 👍

[PierreZ]

Can the path be a parameter from cli with this default?

[PierreZ]

Thanks a lot for your PR and congrats! 🚀🚀🚀