Support for if/else-if/else conditionals

[brandonscript]

Not sure how we're going to implement this yet, but the mutually exclusive if-statements are a bit of a challenge at times. Should support the following syntaxes:

if :: condition :: responseIfTrue                      // this exists already
if :: condition :: responseIfTrue :: responseIfFalse   // new

and

if :: condition :: responseIfTrue
elseif :: condition :: responseIfTrue
elseif :: condition :: responseIfTrue
else :: defaultResponse

[brandonscript]

Added the following syntax in 920b830:

if :: condition :: responseIfTrue :: responseIfFalse