Allow default header and query versioning

Allow people to specify a default version when versioning their API
endpoints via ACCEPT headers or query parameters. Example:

```ruby
class Versioning < Crepe::API
  version with: :header, vendor: :example, default: :v2

  # Define endpoints
end
```

Of course, this doesn't make sense with path versioning, so I've left
that out.

Signed-off-by: David Celis <me@davidcel.is>