Filter: remove attributes from code blocks (hugo)

[cagix]

In order to get proper syntax highlighting when using Hugo,

• remove attributes from codeblocks, and
• remove classes and attributes from inline code.

---

Examples:

``` {.haskell size="tiny"}
class Monad m where
    (>>=) :: m a -> (a -> m b) -> m b
    return :: a -> m a
```

and

`void`{.c}

would not be recognised by Hugo. Thus this PR introduces some preprocessing and the hugo.lua filter will emit

``` haskell
class Monad m where
    (>>=) :: m a -> (a -> m b) -> m b
    return :: a -> m a
```

and

`void`

---

fixes #83