Filters

Another thing I didn't see if is possible, It would be nice if we could create filters and register them.
Something like:

```
$jade = new \Jade\Jade();
$jade->register($callable);
```

And use them:

```
p
    :custom_filter
        My filtered text
```

also this would be great:

```
p
    a #{'hello' | translate}
```
