enhanced template token rendering / custom filter

Hi Bong.
We had the question how to format little text vars in template in IRC yesterday. I thought it would be nice to use the format flag for template tokens here to do so, just like `{{'<h1>{0}</h1>',@title | format }}`.

**The problem:**
This works great, but when we put that format string into the hive as well and do `{{@formatAsHeadline,@title | format }}`, we run into trouble with the auto-escaping (`ESCAPE`), because the format-string will be escaped before the format happens in template rendering. So we would end up with a result like `&lt;h1&gt;Headline&lt;/h1&gt;`, which is not what we wanted. 
Of cause we don't want to turn off the auto-escaping, nor stupid hacks,.. so i came up with an idea: 
why not make the token suffixes / modifiers chainable.. 

**The idea:**
currently we have `{{ expr | esc }}`, `{{ expr | raw }}` and `{{ expr, arg1,... | format }}`. Well this problem would be fixed if we could do something like `{{ expr, arg1,... | format raw }}` - first do the format, then convert to raw - awesome!

**The solution:**
I just made some slight modifications to the Preview class that handles this part. And i also tried to make it **extendable**! So you could add own custom filters in a similar way Template->extend works. New "Modifiers" could be added with `\Preview::instance()->modify('crop', '\App\Modifiers->crop');` (i was not sure about the method name yet, but you get the idea)

So if you like, you're now able to add your own filters i.e. like a crop filter to crop and escape your text in templates easiely: `{{ @text, 150 | crop esc }}`.
I guess there are also many other situations where this could be useful, or what do you think folks?

I used Base->relay in the template to get this chain working.. so the first modifier in line starts, and returns the result to the next one.

The only thing i could not include yet: `Template->extend` is able to save a callback / lambda function... this does not work here, because custom tags are build at render-time but token modifiers impact on execution of the pre-rendered / cached template. And at this point the lambda functions could be away, and putting a Closure inline into the pre-render template does not work... at least, i have no idea atm.