Support alignment modifiers

I've found myself wanting something like (might be buggy, but you get the idea)

```
aligned :: Int -> Get a -> Get a
aligned n g = do
br <- fromIntegral <$> bytesRead
skip $ n - br `rem` n
g
```

Might it be worth adding to the library, with a `Builder`/`Put` counterpart? The `Builder` side of things would require more changes to make it work than what I wrote above, but it's not all that hard.