Add default value to video site options

Why not, right? and it allows a simpler interface to access the
subclasses directly, eg:

```ruby
FilmSnob::YouTube.new('https://www.youtube.com/watch?v=kwYpXVIqzso').html
```

instead of

```ruby
FilmSnob::YouTube.new('https://www.youtube.com/watch?v=kwYpXVIqzso', {}).html
```

That's nice, not having to provide an empty hash

Noticed this while reviewing old issues, specifically https://github.com/maxjacobson/film_snob/issues/26 which suggests encouraging people to use this interface