Proposal: Over-riding values

The `beforeSet` hook is well-positioned to modify whatever value is being set
on a given property. This could be achieved by using the return value of that
user-provided callback function. Doing so could make the library more
cumbersome, though--users who simply wish to observe "set" operations will be
forced to include an explicit `return` statement. A separate API method may be
in order, provided the use case seems sound.