Option for reader to only check nil

[evanthegrayt]

Currently debating if bool_reader should have a way to only check against nil. Currently, it will return false if the ivar value is nil or false, but sometimes you need to just see if the value is set, even if it's false. This will fail in that instance.

It could be argued that the user can just define a condition, but this seems like something that would be used enough to make an actual option for.

In fact, you could even argue that this should be the default.

To counter the previous points, it could be argued that if your value is expected to be a boolean (false), then you'll mostly likely be using bool_accessor anyway.

I'm going to ponder this for a bit. Feedback is welcome.