Add support for additional strongly-typed validation context

[craigfowler]

I'd like to add the capability for validators and validation rules to receive a strongly-typed object which contains validation context. This is a per-validation-run object which is specified by the consumer. It would be used to provide arbitrary data to the validation process.

For example, this context object could be used to supply some pre-fetched data from a database, which will be used/reused in order to run many rules. This could mean that rules do not need to repeatedly get this data for themselves.

This is not logically part of the object being validated, but it is supplementary data related to the validation process.