Add the possibility to get current url

This PR allows you to get the current url with our smarty plugin.

Just use the syntax
```smarty
{url current="1" anotherArg=$value}
```

If you define an argument in the function that already exist in the url, it will override the current one.
Example:

My url is: http://example.com/?foo=1&bar=2

the result of:
{url current="1" bar=3} will be http://example.com/?foo=1&bar=3