Add conditional blocks to templates

Adds conditional blocks to the curly language.  I decided to go with Moustache's syntax, i.e.

```
{{#true?}}
something
{{/true?}}
```

I'm not sure if this is acceptable, since there was no `CONTRIBUTING.md`, but I added the tests for the scanner and compiler.  This should address #32, unfortunately, it doesn't block normal references (i.e. `{{true?}}`) from using a conditional.