Feature: PSR4 compatible

[ceesvanegmond]

Fixes #43

[macx]

@omz13 Please merge it. Easy task.

[MaluNoPeleke]

How to install it with Composer? I added it as an additional repository but no luck:

composer.json:

    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/ceesvanegmond/kirby3-xmlsitemap"
        }
    ],
    "require": {
        "getkirby/composer-installer": "^1.1",
        "ceesvanegmond/kirby3-xmlsitemap": "^1.2.1",

[b0rt]

to use the fork of ceesvanegmond modify composer.json like

// First we want to change the version number to the targets branch name, prefixed with “dev-”, like so:
 "require": {
    "omz13/kirby3-xmlsitemap": "dev-master",
  },
// then tell composer where to pull 'master' from  
  "repositories": [
    {
      "type": "vcs",
      "url": "https://github.com/ceesvanegmond/kirby3-xmlsitemap"
    }
  ],

be mindful: off course you loose the sync to the versions on omz13 package!

[macx]

@b0rt Don't care about losing the sync: The last commit was 2019, it's dead.