Add if flow

Adds something similar to what is discussed here http://joseoncode.com/2012/06/24/messing-with-cps-in-js/ and what we talked about https://github.com/Clever/clever-common/pull/68

Allows us to write that code to https://github.com/Clever/clever-common/pull/69
I thought that `if` would be a reserved word everywhere. I see it can be used when defining something inside a module?