Message bundle validation removal

Message bundle validation has been removed entirely (along with the `validate` option) and optimization has been simplified:

- Message bundles are no longer optimized when loaded via the now-removed `optimize` option
- `int17.optimize(messages)` can be used by build systems to remove meta data (i.e. descriptions and examples)
- `int17.parse(contents)` can be used to build message bundles from strings (simply masks call to `JSON.parse`)
- Placeholder lookups can be optimized by disabled the `ignoreCase` option (enabled by default)