Add generate method to MetaTags, Schema and Robots

[MuhammadSaim]

Twig and other templating engines do not convert objects to a string add method generate() to generate string on demand anywhere in the template.

// schema
$schema->generate();

// metatags
$metatags->generate();

// robots
$robots->generate();

[melbahja]

We don't need a generate func since we have __toString method

[marclaporte]

@MuhammadSaim Thoughts?

[MuhammadSaim]

@marclaporte I was working on the Codeigniter 3 with Twig integrated and using this package need to return the string because Twig is not converting the object to a string with __toString(). Hence, I need a method that generates the string and I can pass it to the template engine and render it as a raw HTML. That's why I propose this.

Sorry new to OpenSource just learning the norms and rules of PRs Thanks