Feature/global sitemap ignore

Added **sitemap_ignore** als global page setting.
Therefore I changed ``___install`` and ``___uninstall``.

And I added two hooks to handle this:
```php
    $this->addHookAfter('ProcessPageEdit::buildFormSettings', $this, 'addIgnoreXML');
    $this->addHookAfter('Pages::save', $this, 'saveIgnoreXML');
```

The main functionality stayed unchanged.

And I added some content to the ReadMe file.