Default options

Changes so far:
- `MultiJson` module has separate `load_options` and `dump_options` getter/setter
- `default_options` is deprecared and sets both  `dump_options` and `load_options`
- Options accept hashes, lambdas and `to_hash`-objects
- Each adapter supports it's own `load_options` and `dump_options` getters/setters and it's priority is higher than global