Mechanism/API to specify a default conversion for any property of a given type in the model

[ajcvickers]

Value conversions were introduced by #242. Currently conversions are only set per-property, although bulk configuration can be used at the end of OnModelCreating in the normal way. This issue is about adding some kind of mechanism to set the conversion for all properties of a given type in the model. This could be through configuration on the model, or it could be via improved bulk config APIs.

[ajcvickers]

Note that the converter isn't really for "every property" even though it will also have that effect. It should also be for any time the CLR type is used in the query--that is, it is really a custom type mapping setup by the application.