If-Else block allow multiple else blocks

[sshailabh]

In conditional block, multiple else are allowed in the template. This should throw an error on compilation. Sample example:

{{#if author}}
 If block
{{else}}
Else 1
{{else}}
Else 2
{{/if}}

Handlebar.js is able to catch multiple else statements. Below is the error generated there.

[sshailabh]

Fixing the issue with this pr: #1154
Here, changed the parser grammar to disallow multiple else statements. Please check the description for more details.