Inverted Loops {^tag}

This adds support for using inverted loops similar to Mustache:

```
Template:

{{#repo}}
  <b>{{name}}</b>
{{/repo}}
{{^repo}}
  No repos :(
{{/repo}}
Hash:

{
  "repo": []
}
Output:

No repos :(
```

I realize that loops can be rather complicated whether it is a dash loop, or it is working on an inner tag or outer tag, so please let me know if anything needs more work, or if I should write more tests!

#37