Add helper function appendSite in class TwigExtension.

Hi there!
I wrote a helper function called appendSiteUrl for the TwigExtension class.
The function is used to append parameters to the site url.
I thought this function might be useful.

Example usage:

{{ appendSiteUrl('/posts', 10)}}	=> .. /posts/10
{{ appendSiteUrl('/posts/', 10)}}	=> .. /posts/10

Cheers,
Alexander