Feature/global sitemap ignore

[justb3a]

Added sitemap_ignore als global page setting.
Therefore I changed ___install and ___uninstall.

And I added two hooks to handle this:

    $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 ;)

[sunlix]

Hey @Notanotherdotcom ,

can you have a look on this pull request?
I like it much and it would be nice to be in the core module to reach all processwire users via the built in module manager/updating process.

I like the possibility that this setting is automatically available without setting up each template manually with the field. This makes more sense in the way the user use this.

regards,
Sven