Add support for conditionals

[alexschneider]

Right now, this allows for the following:

if true: out("false")
else if false:
  out("true")
else: out("maybe")

because support for that is trivial to add (unlike we feared). If anyone doesn't like that, we can easily only only allow blocks for combined if, else if, else expressions, and only allow the single line for if expressions alone.

[alexschneider]

To save headaches in the future, if people install new modules, the npm-shrinkwrap.json needs to be updated!

[alexschneider]

@rachelriv Updated, please merge

[rachelriv]

@alexschneider LGTM but can you please rebase so it can be merged automatically?

[alexschneider]

@rachelriv done

[rachelriv]

It looks like one of your parser tests if failing. This should be good once all tests are passing.

[rachelriv]

LGTM.