Add elseif

[cdepillabout]

This adds support for the elseif control statement.

[cdepillabout]

The last 3 commits (6f17b81, 9ee4df3, and 0f995e6) add an example executable showing off how to use heterocephalus.

I've also added a section to the README about how it can be used to check the generated Haskell code.

[arowM]

Thanks, @cdepillabout .
Could you please move last three commits to separate repository?

I recommend you to publish the contents of the last three commits on your own blog because of following two reasons.
First, because they are not heterocephalus-specific problem, the README.md of heterocephalus is not an appropriate place to inform that.
Second, it is so valuable know how that you should make them as your own achievement.

[cdepillabout]

@arowM Okay, I'll go ahead and put them up on my own blog.

I've edited the PR to just include the first two commits.