Allow scala blocks in filters

[h3adache]

Allowing something like - for(i <- 42 to 46) in filters is extremely helpful when having to create arbitrary blocks of javascript or css based on collections. Currently this is not supported and requires you do hacks such as:

- for(i <- 42 to 46)
    :!javascript
        callSomethingUsing(#{i})

Or having to mix :! filters and non : by saving the collection being iterated to a global javascript constant first. Neither is very desirable.