Add support for else if

[thekid]

Example:

<!-- Now possible -->
{{#if template}}
  Template
{{else if hardwired}}
  Hardwired
{{else}}
  Cannot determine type
{{/if}}

<!-- Essentially the same as -->
{{#if template}}
  Template
{{else}}
  {{#if hardwired}}
    Hardwired
  {{else}}
    Cannot determine type
  {{/if}}
{{/if}}

See handlebars-lang/handlebars.js#892

[thekid]

Released in https://github.com/xp-forge/handlebars/releases/tag/v6.0.0