Add lower-case @if macro

[dalum]

This adds an @if macro as an alias for to the upper-case @If.

[wookay]

thanks. but I think that it needs more consensus.
I still think @If is fine enough.

[dalum]

That's totally alright. I personally found @If quite surprising, so I thought it was probably chosen as a compromise because if is a reserved keyword, and you can't just define a macro with that name the standard way.