Default options

The code is ugly and not very DRY — this is Proof-of-Concept pull-request mainly for discussion.

Changes so far:
- `MultiJson` module has separate `load_options` and `dump_options` getter/setter
- Old `default_options` is aliased to `dump_options` (turned out we never used it for load)
- These getters/setters accept Hashes, Objects implementing `to_hash` or lambdas
- Each adapter supports it's own `load_options` and `dump_options` getters/setters and it's priority is higher than global