Accept a block for attr_initialize and friends

[jferris]

The block is useful when using attr_extras in conjunction with classes like SimpleDelegator from Ruby's standard library. It's also helpful for setting up memoized lookup hashes.

[henrik]

I've thought about something like this. Looks simple and clean and you seem to have a specific use for it, so let's do it.

Could you also update README to document it?

[jferris]

Thanks! I updated the README in 346999b.

[henrik]

Thank you!