Boost up loading speed

Gem loading speed was very low, so I moved filters dependencies within `add_filter` block (a kind of deferred loading).

Loading speed before:

```
$ curl -fsSL https://gist.github.com/raw/5022636/benchmark.rb | ruby

Gem                            Time(sec)     Pct %
--------------------------------------------------
rails                             9.3590     42.4%
auto_html                         1.1122      5.0%
```

and after my improvements

```
auto_html                         0.2074      1.0%
```

Now your app will start a bit faster.