Make Mustache_LambdaHelper invokable

[Svish]

Suggestion to make the Mustache_LambdaHelper class "invokable"? Would make lambda's and such a tiny bit cleaner, which I like, and very simple to do:

Added to Mustache_LambdaHelper

/**
 * Alias of render(string).
 */
public function __invoke($string)
{
    return $this->render($string);
}

Should be backwards compatible and works for PHP>=5.3. The wiki says the mustache code works for PHP>=5.2, but don't think this would break that. It just wouldn't be usable for that version.

Example usage

$engine->render('foobar', [
    'upper' => function($text, $render)
        {
            return strtoupper($render($text));
        },
    ]);

[bobthecow]

👍 This has been added to the dev branch and will go out in the next release.