Allows users to chain multiple helpers together using their inverse callbacks. I.e.
```
{{#if foo}}
{{else if bar}}
{{else}}
{{/if}}
```
The control flow here effectively causes the helpers to be nested. The above is actually syntactic sugar for this:
```
{{#if foo}}
{{else}}
{{#if bar}}
{{else}}
{{/if}}
{{/if}}
```
Any helper may be used in this manner, the only requirement is they support normal calls and inverse calls.
Introduces a breaking change in that `{{else foo}}` may no longer be used as a root level operator. Instead `{{^foo}}` must be used.
Fixes#72.
kpdecker
4282668d47