Added support for {{helper(syntax, "with", "parentheses", and="commas")}}

At the company I work for, we use some other (PHP) templating systems with slightly different syntax for helpers, e.g.
```
{{helper(syntax, "with", "parentheses")}}
```
We've build some tools and workflows around this, depending on this syntax (e.g. non-tech savvy translators are used to this). We want to start using handlebars.js, but the parentheses/comma-less syntax of helpers is kind of deal breaker for the translators.


The commit(s) related to this pull request extend the grammar of the handlebars templates to support the syntax
```
{{helper(syntax, "with", "parentheses", and="commas")}}
```


Would there be any interest to include this syntax extension in the official handlebars.js library?
Or are we condemned to maintain or own fork? :)